ABSTRACT
X-ray images of two small dark clouds, L1455 and L723, have been obtained with the EPIC cameras on board the XMM-Newton telescope. Both regions contain multiple independent but overlapping bipolar flows in high-velocity CO, a collection of Herbig-Haro objects, and a number of filaments and knots of H2 emission. The field of view in each cloud was centered on the confluence of the nearly orthogonal CO flows. More than three dozen compact X-ray sources were detected. However, the Class 0/I protostars thought to be the driving sources of the flows in both clouds were not detected. Strong emission was observed from RNO 15, an optically visible, nebulous T Tauri star in the dense core of L1455. A thermal plasma model for the X-ray spectrum of RNO 15 gave a luminosity of LX ∼ 1031.2 erg s−1. The heavily reddened Class II star L1455 IRS 5 in the same region was detected as a weak X-ray source. No emission was detected from IRS 1 or IRS 4, which have been suggested as the launch sites for separate outflows in L1455. X-ray emission was observed from IRAS 19156+1906 in L723. The thermal radio source VLA 2, which is offset ∼10'' east of the IRAS position and is considered the most likely energy source for the strong east–west flow in L723, was not detected. The source of the more narrowly collimated north–south flow in L723 is unknown but may be an embedded infrared object at the X-ray and IRAS position.
1. INTRODUCTION
The birth of a young star in a molecular cloud is often accompanied by an outflow of high-velocity gas in the form of a high-speed wind or collimated jet. Outflows are most commonly observed in the spatially extended emission of CO, but may also appear as parsec-scale optical or near-infrared jets and counterjets, the so-called Herbig-Haro (HH) objects. Most young stellar objects (YSOs) with outflows are faint or even invisible at short wavelengths and display a very steep spectral energy distribution (SED) at far-infrared (FIR) and submillimeter wavelengths. This is an indication that the central stars are still deeply embedded in their birth clouds and are concealed from view by their dense circumstellar disks and envelopes of infalling material. Commonly referred to as Class I protostars, such objects are thought to be in an early stage of evolution just preceding the T Tauri star (TTS) or Class II phase.
The question of how the Class I YSOs, or the even younger and more deeply embedded Class 0 objects, drive such outflows has not been settled yet. It is now widely accepted that the outflows are centrifugally driven and that magnetic fields play a key role in collimating and driving the mass loss (for two recent reviews of the theory, see Shu et al. 2000 and Königl & Pudritz 2000). In addition, the magnetic field of a YSO may also act to accelerate particles that generate intense soft X-ray emission (Hayashi et al. 1996; Goodson et al. 1997). Depending, then, on the amount of extinction that arises from gas and dust along the line of sight, the X-ray emission of a YSO may be strong enough to be detected by the X-ray telescopes on XMM or Chandra. The detection of a YSO that is heavily obscured is much more likely, of course, if it undergoes a powerful X-ray outburst during the observation, as has happened on rare occasions (e.g., Grosso et al. 2005; Hamaguchi et al. 2005; Simon & Dahm 2005).
To date, nearly all of the existing X-ray surveys of Class I YSOs with high-speed outflows have been carried out in massive, dense clouds like Orion, which give birth to hundreds of young stars over their lifetime. Comparable investigations of the many small clouds (∼102 M☉) that ultimately form aggregates of just a few (or a few dozen) stars have been the exception. In one such study of the low-mass L1251 cloud, I reported the discovery of weak X-ray emission from IRAS 22376+7455, an extreme Class I YSO that is thought to be the origin of a compact molecular outflow and a thermal radio jet (Simon 2006). That initial detection with XMM was recently confirmed in a follow-up observation with Chandra, which fortuitously captured a very bright X-ray flare of the embedded protostar (Simon 2009). In this paper, I present the results of the first X-ray observations of two additional small dark clouds, L1455 and L723. Both regions (along with L1251) have been observed by the Spitzer Space Telescope (SST) in the SST Legacy project "From Molecular Cores to Planet-Forming Disks" (c2d; Evans et al. 2003). CO maps of the high-speed outflows in both clouds reveal a complex structure of multiple overlapping and intersecting bipolar flows. It is not known for certain which of the young objects embedded in the clouds are responsible for driving the observed outflows. A primary goal of the present investigation was to search for X-ray emission from areas around the CO emission cores that might help to identify the origin of the large-scale, high-velocity flows in both clouds.
2. OBSERVATIONS AND DATA ANALYSIS
2.1. Description of the Sample
The first of the two regions, L1455, contains three low-luminosity IRAS sources, two of them in the central portion of the cloud where the most intense activity is concentrated (Bachiller & Cernicharo 1986). One of the IRAS sources, IRAS 03247+3001, coincides with a faint nebulous T Tau star, RNO 15 (Cohen & Kuhi 1979; Cohen 1980) and the Class II submillimeter peak L1455 IRS 2 (Wu et al. 2007). The other, IRAS 03245+3002, coincides with the Class I FIR and submillimeter source, L1455 IRS 1 (Wu et al. 2007; Shirley et al. 2000). The FIR source of Davidson & Jaffe (1984) lies slightly east of IRS 1, but is very likely the same object. Both IRAS sources have near-infrared counterparts in the c2d Spitzer IRAC images of L1455 (Jørgensen et al. 2006). Two other sources are prominent in the IRAC images: IRS 4, which is a bright Class I or Class 0 submillimeter source (Wu et al. 2007; Shirley et al. 2000; Hatchell et al. 2007), and IRS 5, which is a heavily reddened Class II star that is also known as L1455 FIR 2 (Tapia et al. 1997). The CO map of Goldsmith et al. (1984) traces out the high-velocity gas in this main section of L1455. It shows an elongated structure with red- and blue-shifted peaks that are well separated along an northwest–southeast axis, which defines the direction of the outflow. The source of the outflow has not been firmly established; however, based on the geometry of the flow and the relative locations of the known YSOs, IRS 4 is a strong candidate, as it lies closest to the center line of the flow.
Bally et al. (1997) have identified a number of HH knots and filaments from Hα and [S ii] images of L1455. Some of the same features are also visible in the near-infrared H2 v = 1–0 S(1) maps of the central part of the cloud made by Davis et al. (1997a). HH 318, among the brightest of the Hα knots, and three other H2 knots are closely aligned and coincide with a narrow northeast–southwest ridge of CO 3–2 emission (Davis et al. 1997b). IRS 1 lies along the same ridge, centered between the lobes of red-shifted CO to the northeast and blue-shifted gas to the southwest. IRS 1 thus appears to be the origin of a separate, narrowly collimated outflow of its own. This second flow overlaps but is independent of, and nearly orthogonal to, the CO flow that is possibly launched by IRS 4 from virtually the same location in the cloud.
The L723 star-forming region, the second dark cloud in the present study, is rather inconspicuous at optical wavelengths. It contains a single low-luminosity Class 0 source, IRAS 19156+1906 (Reipurth et al. 1993; Shirley et al. 2000). Starting with the original CO map of Goldsmith et al. (1984), L723 has been surveyed in increasing detail by Avery et al. (1990); Hirano et al. (1998); and Lee et al. (2002). The region has also been extensively mapped in CS (Hirano et al. 1998) and NH3 (Girart et al. 1997). Shirley et al. (2000) mapped the area using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) and found a single 450 μ and 850 μ peak coinciding with the IRAS source. Later work by Young et al. (2006), also using SCUBA, placed the 850 μ peak 75 farther east (but still within the IRAS error ellipse) at the position of an elongated radio jet, VLA 2 (Anglada et al. 1996). Another radio source in the same area, VLA 1, is unresolved and appears to be an extragalactic source that is unrelated to L723 (Anglada et al. 1996). The FIR source of Davidson (1987), L723 FIR, also coincides with VLA 2. Dartois et al. (2005) reported the detection of a very large column of CO2 ice from spectra taken with the Spitzer Infrared Spectrograph, apparently in the direction of VLA 2. However, the exact coordinates of the observation were not given and cannot be determined from that work.
The interpretation of the CO observations of L723 has been controversial. Some authors have favored a single bipolar outflow from either the IRAS position or the radio position in the cloud core, but others have argued for a pair of independent bipolar flows that emerge from a common center there. The highest resolution CO maps reveal blue-shifted gas east and north of the cloud core and red-shifted gas both west and south of it (Figure 5, Hirano et al. 1998; Figure 1, Lee et al. 2002). Avery et al. (1990) have interpreted the morphology of the CO maps in terms of a single bipolar outflow whose CO emission follows the walls of a cavity that has been hollowed out by a wide-angle wind from the IRAS source. Hirano et al. (1998) have proposed a similar shell model but favor an origin from the VLA radio position. Lee et al. (2002) as well as Girart et al. (1997) prefer two independent flows that originate from the cloud core, one with a wide opening angle in the east–west direction (in position angle P.A.∼100 deg), the other a more highly collimated flow in the north–south direction (in P.A.∼30 deg). Girart et al. identify the wide east–west flow with VLA 2 and attribute the narrow north–south flow to a second, thus far undetected driving source at the location of a prominent hotspot in their NH3 map (Figure 8, Girart et al. 1997), which lies within 3'' of the IRAS source. Lee et al. (2002) have suggested that both flows originate from the vicinity of the radio position, the east–west outflow from VLA 2 itself, the north–south flow possibly from an unresolved binary companion of VLA 2. Using the VLA in its high-resolution A configuration, Carrasco-González et al. (2008) have resolved the extended emission of VLA 2 and confirmed that the source is, in fact, a close radio binary. It is quite possible, therefore, that the structure of the quadrupolar flow in L723 is the result of separate flows from the two binary components of VLA 2, one outflow emerging in the north–south direction, the other east–west.
2.2. The XMM Observations
Table 1 provides the details of both XMM pointings. L1455 was observed for an exposure time of 44 kiloseconds (ks) during revolution 1399 on 2007 July 31. The observation ID is 0503670101. The aim point of the observation was centered on L1455 FIR, the FIR source of Davidson & Jaffe (1984). With that orientation, the young emission-line star RNO 15 and its FIR counterpart, IRAS 03247+3001, were placed within 2' of the optical axis of the telescope, where the best images are formed. The EPIC pn camera (Strüder et al. 2001) and the two MOS cameras (Turner et al. 2001) were operated in full-window mode, using the medium filter. L723 was observed with XMM on 2007 May 9 during revolution 1358 for a nominal exposure time of 39 ks. The observation ID is 0503670201. The field of view was centered on L723 FIR, the FIR source of Davidson (1987), and on the adjacent radio source VLA 2 (Anglada et al. 1996). The EPIC cameras were run in full-window mode. The medium filter was used.
Table 1. XMM-Newton Observations
Effective Exposure Timeb | |||||||
---|---|---|---|---|---|---|---|
Field | Observation ID | Date of Observation | Orbital Revolution | R.A. (J2000.0)a | Decl. (J2000.0)a | pn | MOS |
L1455 | 0503670101 | 2007-07-31 | 1399 | 03 27 40.0 | +30 13 00.0 | 24.7 | 31.0 |
L723 | 0503670201 | 2007-05-09 | 1358 | 19 17 54.0 | +19 12 20.0 | 24.3 | 31.2 |
Notes. aUnits of right ascension are hours, minutes, and seconds. Units of declination are degrees, arcminutes, and arcseconds. bExposure times in ks. The nominal length of each observation was 43.5 and 38.8 ks, respectively.
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To analyze both EPIC data sets, I used version 7.1 of the XMM Science Analysis Software (SAS; Watson et al. 2001) and a complete set of updated calibration files, which were downloaded from the XMM Science Operations Center (dated 2007 November 13). The SAS epchain and emchain tasks were used to create X-ray event lists for each camera. The lists were subsequently filtered using the SAS xmmselect task to pick out the photon events with pulse height energies in the broad 0.3–10 keV energy band. Events at higher or lower energy outside that band were discarded. Following the data analysis threads described in the XMM data analysis ABC Guide,1 I retained only the pixel events that were assigned PATTERN values of 0–12 for the MOS and 0–4 for the pn. Hot and other bad pixels (i.e., those outside the field of view or close to the edges of the CCD chips, etc.) were removed by filtering out poor-quality events that were tagged with FLAG ≠0 during the normal pipeline processing.
Light curves were extracted in the 10–15 keV energy band from source-free images in each camera in order to remove any intervals of flaring particle background or other times marked by high background emission. Both observations experienced a moderately higher than average X-ray background level in the 0.3–10 keV energy band for most of their scheduled exposure time. The event files for both data sets were therefore filtered using slightly higher than normal rejection levels in order to identify the "good time intervals" of low background in the light curves, 0.35 count s−1 for the two MOS cameras and 2.5 count s−1 for the pn. Photon events that were recorded during the rejected segments of each exposure were eliminated and ignored in the rest of the analysis. To identify likely sources in the EPIC images, I ran the SAS edetect_chain science thread for each observation, searching all three cameras simultaneously. Separate source lists were compiled for a soft energy band of 0.5–2.0 keV, a hard energy band of 2.0–7.5 keV, and the total energy band of 0.3–10 keV. The three independent source lists were edited to remove spurious entries (e.g., false detections along the edges of the EPIC detectors), merged, and then purged of duplications. A final source list was determined for each observation after careful inspection and measurement of the individual broadband EPIC images.
The X-ray counts for each source were measured using the funcnts tool in the FUNTOOLS software package from the Smithsonian Astrophysical Observatory. For sources within 7' of the optical axis, I used a circular extraction cell of radius r = 8'' so as to enclose at least 50% of the energy in the local point-spread function (PSF). For sources located farther off-axis, I chose a larger radius, r = 15'', which also ensured an enclosed energy fraction of 50% or more. The measured counts in each case were extrapolated to 100% enclosed energy using the analytical expression given by Simon & Dahm (2005) and corrected for background emission, as estimated from a nearby, offset background region with r ⩾ 40''. To compute the X-ray count rates for individual sources, I derived exposure maps for each camera in the 0.3–10 keV energy band via the SAS eexpmap task. The exposure maps account for vignetting related changes in the effective response of the telescope across the field of view. Optical and near-infrared identifications were established for the X-ray sources by visually comparing the XMM images with images from the Digitized Sky Survey (DSS) of the Space Telescope Science Institute (STScI) and The Two Micron All Sky Survey (2MASS) Point Source Catalog, and also by cross-matching the pn and MOS image coordinates with entries in the SIMBAD database.
Table 2 summarizes the results of the X-ray photometry for L1455. For completeness I present the measurements from the entire XMM field of view, although the discussion below concentrates on the cluster of YSOs in the cloud core at the center of the field. Results for L723, also the full set of measurements, follow in Table 3. An identification number for each source is provided in Column (1) of both tables. Listed next is the name of the source in IAU format, composed in the usual fashion from its right ascension and declination (J2000.0). Column (3) gives the angular offset of the source from the bore sight of the XMM pointing. The name of any optical counterpart appears in Column (4). Names with a 2G prefix denote entries from the second edition of the Hubble Space Telescope Guide Star Catalog (GSC; Lasker et al. 2008). The offset between the XMM centroid position and the position of the nearest optical counterpart is shown in Column (5). The next two columns specify the effective exposure times for the source in the pn and MOS cameras. Columns (8) and (9) list the background-corrected source count rates and their 1σ statistical errors for the pn camera and the average of the two MOS cameras, respectively. The remaining columns in the tables list the successful matches of the XMM sources with the 2MASS catalog and the corresponding JHKs photometry from the SIMBAD database.
Table 2. X-Ray Detections in L1455
Exposure Time (ks)b | Count Ratec | 2MASS Photometry | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Source (1) | XMMU (2) | θa (arcmin) (3) | Identification (4) | Offset (arcsec) (5) | PN (6) | MOS (7) | PN (8) | MOS (9) | 2MASS (10) | Ks (11) | J − H (12) | H − Ks (13) |
1 ...... | J032643.8+300823 | 13.0 | NVSS J032644+300823 | 5.3 | 7.8 | 9.0 | 16.93 ± 3.05 | 4.80 ± 0.97 | ... | ... | ... | ... |
2 ...... | J032645.9+301102 | 11.8 | ... | ... | 9.2 | 10.9 | 9.30 ± 2.27 | 3.40 ± 1.02 | ... | ... | ... | ... |
3 ...... | J032648.0+301754 | 12.3 | ... | ... | 9.1 | 10.2 | 12.39 ± 2.46 | 4.83 ± 0.87 | ... | ... | ... | ... |
4 ...... | J032700.8+300440 | 11.9 | ... | ... | 8.7 | 10.5 | 29.19 ± 3.13 | 13.20 ± 1.19 | ... | ... | ... | ... |
5 ...... | J032713.4+300925 | 6.8 | USNO-B1.0 1201-0042206 | 1.1 | 8.9d | 19.1 | 6.81 ± 1.94d | 1.83 ± 0.41 | J03271356+3009252e | 13.925 | 0.638 | 0.223 |
6 ...... | J032716.4+301444 | 5.4 | ... | ... | 18.3 | 23.2 | 3.96 ± 0.94 | 1.35 ± 0.31 | ... | ... | ... | ... |
7 ...... | J032720.8+302104 | 9.1 | ... | ... | 13.5 | 18.1 | 5.45 ± 1.52 | 2.81 ± 0.75 | ... | ... | ... | ... |
8 ...... | J032738.1+301358 | 1.0 | L1455–IRS 5 | 1.9 | 24.3 | 30.8 | 3.49 ± 0.65 | 1.38 ± 0.23 | J03273825+3013585 | 10.658 | 4.311 | 2.111 |
9 ...... | J032742.2+301138 | 1.4 | NVSS J032742+301140 | 2.1 | 20.2 | 27.0 | 2.32 ± 0.68 | 0.66 ± 0.21 | ... | ... | ... | ... |
10 ..... | J032747.6+301204 | 1.9 | CoKu N1333 Star 2 = RNO 15 | 0.9 | 23.6 | 28.2 | 50.48 ± 2.10 | 22.39 ± 0.89 | J03274767+3012043 | 8.495 | 1.639 | 0.953 |
11 ..... | J032754.0+302232 | 10.0 | ... | ... | 13.1 | 17.9 | 6.53 ± 1.55 | 2.03 ± 0.62 | ... | ... | ... | ... |
12 ..... | J032754.2+301948 | 7.5 | ... | ... | 16.3 | 22.5 | 5.98 ± 1.22 | 1.59 ± 0.50 | ... | ... | ... | ... |
13 ..... | J032754.2+301716 | 5.3 | ... | ... | 20.2 | 25.9 | 2.30 ± 0.77 | 1.27 ± 0.28 | ... | ... | ... | ... |
14 ..... | J032759.7+300119 | 12.4 | ... | ... | 8.9 | 11.2 | 17.93 ± 2.76 | 9.73 ± 1.04 | ... | ... | ... | ... |
15 ..... | J032800.0+300847 | 6.0 | 2G N3330301-5194 | 0.7 | 18.2 | 22.6 | 3.30 ± 0.90 | 0.88 ± 0.28 | J03280010+3008469 | 10.326 | 1.081 | 0.616 |
16 ..... | J032801.3+301606 | 5.5 | ... | ... | 20.2 | 25.6 | 3.99 ± 0.87 | 1.10 ± 0.27 | ... | ... | ... | ... |
17 ..... | J032813.8+301404 | 7.4 | ... | ... | 17.5 | 20.4 | 8.62 ± 1.34 | 3.69 ± 0.53 | ... | ... | ... | ... |
18 ..... | J032814.9+300119 | 13.9 | 2G N3330301-436 | 1.5 | 7.8 | ... | 82.38 ± 4.97 | ... | J03281495+3001192e | 9.561 | 0.314 | 0.097 |
19 ..... | J032822.0+301802 | 10.4 | 2G N3330301-6086 | 2.6 | 12.9 | 16.6 | 13.67 ± 1.89 | 3.65 ± 0.78 | J03282215+3018030e | 12.396 | 0.600 | 0.279 |
20 ..... | J032827.4+300459 | 13.0 | ... | ... | 9.0 | 11.6 | 6.24 ± 2.26 | 3.23 ± 0.72 | ... | ... | ... | ... |
21 ..... | J032829.2+301621 | 11.1 | ... | ... | 11.9 | 15.1 | 4.28 ± 1.31 | 1.50 ± 0.49 | ... | ... | ... | ... |
Notes.
aOff-axis angle measured in arcminutes from the aim point of the observation, α(J2000.0) = 03h27m400, δ(J2000.0) = +30°13'00''.
bEffective exposure time based on exposure maps for the 0.3–10.0 keV energy band, which account for the variation in camera sensitivity across the field of view.
cBackground-corrected source count rate in counts ks−1 in the 0.3–10.0 keV energy band for the pn camerga and for the average of the two MOS cameras. The on-source counts were measured through a circular aperture 8'' in radius for θ ⩽ 7', 15'' for θ>7', and were corrected to 100% encircled energy fraction to compensate for the dependence of the throughput of the extraction aperture on its size and the location of the source within the field of view. Background counts were measured through an adjacent circular aperture having a radius of 40''. The cited errors are statistical only. Count rates in the narrower 0.3–8.0 keV band are virtually identical to those given here for the 0.3–10.0 keV range.
dSource located on a bad column or near the edge of a detector chip, or outside the field of view of one or more cameras.
eInfrared photometry consistent with that of field dwarf.
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Table 3. X-Ray Detections in L723
Source | XMMU | θa | Identification | Offset | Exposure Time (ks)b | Count Ratec | 2MASS | 2MASS Photometry | ||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
(arcmin) | (arcsec) | PN | MOS | PN | MOS | Ks | J − H | H − Ks | ||||
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12) | (13) |
1 ...... | J191651.1+191520 | 15.2 | 2G N0230300-295 | 0.8 | 6.7 | ... | 109.22 ± 6.89 | ... | J19165118+1915216e | 8.931 | 0.655 | 0.243 |
2 ...... | J191709.0+191936 | 12.9 | HD 349941 | 2.2 | ... | 11.6 | ... | 5.09 ± 1.38 | J19170920+1919406e | 9.786 | 0.197 | 0.067 |
3 ...... | J191733.1+192334 | 12.3 | 2G N0230303-110 | 0.8 | ... | 12.6 | ... | 13.43 ± 1.70 | J19173325+1923356e | 8.384 | 0.282 | 0.033 |
4 ...... | J191739.5+191432 | 4.1 | 2G N0230300-63182 | 0.7 | 16.2d | 26.7 | 11.31 ± 1.64d | 3.07 ± 0.43 | J19173959+1914330e | 13.459 | 0.579 | 0.288 |
5 ...... | J191747.9+190853 | 3.7 | 2G N0230300-60893 | 1.6 | 19.4 | 25.6 | 6.97 ± 1.24 | 1.56 ± 0.34 | J19174789+1908525e | 12.069 | 0.629 | 0.245 |
6 ...... | J191749.7+191146 | 1.2 | ... | ... | 23.0 | 30.1 | 7.02 ± 1.02 | 2.06 ± 0.31 | ... | ... | ... | ... |
7 ...... | J191752.3+191331 | 1.3 | ... | ... | 23.9 | 30.9 | 10.70 ± 1.14 | 4.12 ± 0.39 | ... | ... | ... | ... |
8 ...... | J191753.0+191216 | 0.2 | 2G N0230300-62415 | 0.7 | 23.9 | 30.9 | 3.92 ± 0.86 | 1.05 ± 0.24 | J19175313+1912163 | 12.118 | 0.858 | 0.327 |
9 ...... | J191753.7+191454 | 2.6 | ... | ... | 23.2 | 29.8 | 3.57 ± 0.96 | 1.22 ± 0.29 | ... | ... | ... | ... |
10 ..... | J191758.2+190518 | 7.1 | ... | ... | ... | 18.8 | ... | 3.78 ± 0.87 | ... | ... | ... | ... |
11 ..... | J191800.8+190916 | 3.5 | 2G N0230300-306 | 2.2 | 20.4 | 26.2 | 4.20 ± 1.07 | 0.80 ± 0.29 | J19180092+1909153e | 10.378 | 0.187 | 0.067 |
12 ..... | J191802.5+192205 | 10.0 | ... | ... | 12.4 | 17.4 | 3.91 ± 2.13 | 1.97 ± 0.57 | ... | ... | ... | ... |
13 ..... | J191809.1+191412 | 4.0 | ... | ... | ... | 27.8 | ... | 1.97 ± 0.35 | ... | ... | ... | ... |
14 ..... | J191809.5+191702 | 6.0 | ... | ... | 19.4 | 24.8 | 5.93 ± 1.34 | 3.30 ± 0.48 | ... | ... | ... | ... |
15 ..... | J191813.4+191638 | 6.3 | 2G N0230303-37152 | 0.8 | 19.0 | 24.0 | 6.79 ± 1.37 | 1.84 ± 0.40 | J19181351+1916379 | >11.654 | <0.535 | ... |
16 ..... | J191820.3+191812 | 8.5 | 2G N0230303-38908 | 1.5 | 15.4 | 19.9 | 6.84 ± 1.74 | 3.01 ± 0.74 | J19182057+1918116e | 12.549 | 0.645 | 0.278 |
17 ..... | J191823.7+190230 | 12.1 | 2G N0230300-329 | 2.0 | 9.0 | 12.1 | 9.60 ± 3.27 | 3.24 ± 1.29 | J19182377+1902300e | 10.464 | 0.382 | 0.103 |
18 ..... | J191824.2+192137 | 11.7 | ... | ... | 11.2 | 13.9 | 20.08 ± 2.97 | 6.95 ± 0.94 | ... | ... | ... | ... |
Notes.
aOff-axis angle measured in arcminutes from the aim point of the observation, α(J2000.0) = 19h17m540, δ(J2000.0) = +19°12'20''.
bEffective exposure time based on exposure maps for the 0.3–10.0 keV energy band, which account for the variation in camera sensitivity across the field of view.
cBackground-corrected source count rate in counts ks−1 in the 0.3–10.0 keV energy band for the PN camera and for the average of the two MOS cameras. The on-source counts were measured through a circular aperture 8'' in radius for θ ⩽ 7', 15'' for θ>7', and were corrected to 100% encircled energy fraction to compensate for the dependence of the throughput of the extraction aperture on its size and the location of the source within the field of view. Background counts were measured through an adjacent circular aperture having a radius of 40''. The cited errors are statistical only. Count rates in the narrower 0.3–8.0 keV band are virtually identical to those given here for the 0.3–10.0 keV range.
dSource located on a bad column or near the edge of a detector chip, or outside the field of view of one or more cameras.
eInfrared photometry consistent with that of field dwarf.
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2.3. A Near-Infrared Color–Color Plot
Six of the 21 X-ray sources detected by XMM in L1455 and 10 of the 18 sources in L723 can be identified with near-infrared sources in the 2MASS survey. The corresponding JHK photometry from 2MASS is plotted in a color–color diagram in Figure 1 after conversion to the CIT system via the transformations given in Carpenter (2001). The open circles denote sources from L1455, those from L723 are plotted as filled symbols. Three X-ray sources plotted in the figure have very red colors and occupy the same part of the color–color plot as highly reddened normal stars and embedded protostars (see, for example, Lada & Adams 1992). The reddest of the three, source 8 in L1455, can be identified with the embedded Class II star known as L1455 IRS 5 (Figure 16, Jørgensen et al. 2006; Figure 7, Wu et al. 2007). Tapia et al. (1997) identified the same object as FIR 2 and from its near-infrared colors derived an optical extinction of Av ≈ 30 mag. The 2MASS photometry plotted in Figure 1 confirms that earlier reddening estimate.
Figure 1. Infrared two-color diagram for X-ray sources with counterparts in the 2MASS Point Source Catalog. Open circles: X-ray detections in L1455 from Table 2. Filled circles: X-ray detections in L723 from Table 3. The solid lines are the dwarf and giant sequences given by Tokunaga (2000). The dot–dashed line is the dereddened classical TTS relation of Meyer et al. (1997). The dashed lines represent the reddening trajectories for giants, main-sequence stars, and TTSs following Mathis (2000). In principle, the foreground reddening of a normal main sequence or giant star can be determined from the color–color plot by projecting its color indices back to one of the unreddened sequences, parallel to the reddening curve. Meyer et al. (1997) have shown that the nonlocal reddening along the line of sight to a TTS can be estimated in the same way by projecting its colors back to the unreddened TTS locus.
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Standard image High-resolution imageThe second reddest object in the infrared color–color diagram is L1455 source 10, which is the classical TTS RNO 15. The optical spectrum of RNO 15 shows variable Hα, O i, and the Ca+ infrared triplet strongly in emission (Cohen & Kuhi 1979; Cohen 1980; Levreault 1988). Br γ also appears in emission (Carr 1990). Following Equation (2) in Muzerolle et al. (1998), the luminosity of the Brackett line suggests a disk accretion rate of ∼few × 10−7M☉ yr−1, which is quite typical for an active CTTS. The mid-infrared spectra of RNO 15 obtained by Spitzer display solid state ice features of H2O and CO2 in absorption (Boogert et al. 2008; Pontoppidan et al. 2008). Both features appear in much greater strength in the spectra of the adjacent submillimeter sources, IRS 1 and IRS 4, and are presumably formed by ice mantles that are attached to very cold dust grains in the cloud. The presence of these ice bands in the spectral scans of RNO 15 is thus a strong indication that this YSO is deeply buried within the L1455 cloud. The location of RNO 15 in the infrared two-color plot in Figure 1 suggests a minimum extinction of Av ≈ 7 mag and leads to a similar conclusion.
There are two other stars of note in the color–color plot. The first is source 15 in L1455, which appears to be a moderately reddened (Av = 2.2 mag) CTTS member of the cloud. Alternatively, if this object is a normal field star that is projected by chance on the line of sight to L1455, its 2MASS colors and magnitudes imply a spectral type of dM5, an extinction of Av = 3.8 mag, and a distance of just 55 pc. At such a small distance, the star lies well in front of L1455 (d = 320 pc; Hatchell et al. 2007) but so close to the Sun that it is difficult to understand how it could suffer the nearly four magnitudes of extinction required in Figure 1. It is much more plausible to conclude that the star is a modestly reddened CTTS that belongs to L1455. The other notable object in Figure 1 is L723 source number 8. This source has a faint optical counterpart with a GSC red magnitude of RF = 17.8. Based on its 2MASS colors, it is either a nearly unreddened M giant in the field or else a very modestly reddened TTS within L723. If it is a giant or a bright giant, the observed GSC and 2MASS K magnitudes together suggest a minimum distance of d = 16 kpc, in which case the star would be much too far away to be detected in X-rays since most late-type giants are intrinsically weak X-ray sources. On the other hand, the 2MASS colors of the source are almost identical to those of the marginal TTS KP2-44 in L1251B (Simon 2006, 2009), which is known to be the origin and driving source of the prominent HH 189 jet in that cloud.
3. RESULTS
3.1. The L1455 Cloud
The brightest source in the XMM observation of L1455, source 18, is at the very edge of the field of view of the EPIC cameras. It can be identified with a bright field star that is located at the outer boundary of the obscuration that defines the cloud. The star was observed to be slightly polarized in the optical linear polarization survey of the Perseus region by Goodman et al. (1990), where it appears as the 15th entry in their Table 3 (Per 15). The 2MASS and GSC colors of the star are compatible with little or no reddening, a spectral type of late F or early G, and a distance of ∼200 pc. The star is clearly not a pre-main-sequence member of the cloud. Two other sources with moderately large count rates, numbers 4 and 14, also lie at the southern edge of the XMM field of view but have no optical or infrared counterparts.
A near-infrared image obtained from the Spitzer public archives of the area surrounding RNO 15 is presented in Figure 2. The image was acquired at a wavelength of 4.5 μ with the IRAC camera in the course of the c2d Legacy program. Various objects of interest are numbered in the figure and identified by name in Table 4. Three XMM sources in the field are marked by plus signs. The location of RNO 15 is denoted by a cross symbol and labeled as object #2. The positions of RNO 15 and X-ray source 10 are obviously in close agreement, differing by less than 1''. The internal error in the X-ray centroid returned by the SAS edetect tool is ⩽05 at the 95% confidence level in both right ascension and declination. For comparison, the pn camera has a pixel size of 4
1 (Strüder et al. 2001) and the MOS cameras a pixel size of 1
1 (Turner et al. 2001). The position of RNO 15 corresponds to the centroid of emission in the IRAC 4.5 μ image. The near-infrared position falls between two 350 μ emission peaks in the map acquired by Wu et al. (2007) using the SHARC-II camera at the Caltech Submillimeter Observatory (CSO) on Mauna Kea, Hawaii, lying somewhat closer to the fainter western extension IRS 2W (i.e., object #3 in Figure 2 and Table 4) than to the main peak IRS 2E (object #4) shown in their map.
Figure 2. Gray-scale Spitzer IRAC 4.5 μ image of the central portion of the L1455 dark cloud. The objects are identified by name in Table 4. RNO 15 (object #2) is the bright star in the lower-left corner of the field of view. The plus symbols represent XMM X-ray sources, and the crosses denote named radio, submillimeter, and optical sources.
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Standard image High-resolution imageTable 4. X–Ray Detections in the CO Molecular Cores
No. | Name | R.A. (J2000) | Decl. (J2000) | Offset ('') | Refs. |
---|---|---|---|---|---|
a) L1455 | |||||
1 ....... | XMM source 10 | 03 27 47.61 | +30 12 04.5 | 0.0 | 1 |
2 ....... | RNO-15 (IRAC 4.5 μ) | 03 27 47.67 | +30 12 04.3 | 0.8 | 1 |
3 ....... | L1455 IRS 2W (350 μ) | 03 27 47.2 | +30 12 04 | 5.3 | 12 |
4 ....... | L1455 IRS 2E (350 μ) | 03 27 48.3 | +30 12 10 | 10.5 | 12 |
5 ....... | L1455 IRS 4 (MIPS 24 μ) | 03 27 43.25 | +30 12 28.9 | 61.6 | 8 |
6 ....... | VLA C | 03 27 42.82 | +30 11 34.2 | 69.3 | 9 |
7 ....... | XMM source 9 | 03 27 42.20 | +30 11 38.5 | 74.8 | 1 |
8 ....... | VLA B | 03 27 41.50 | +30 11 47.0 | 81.1 | 9 |
9 ....... | H2O maser | 03 27 39.84 | +30 12 51.9 | 111.3 | 5 |
10 ..... | HH B | 03 27 41.9 | +30 13 42.2 | 122.6 | 3 |
11 ..... | L1455 IRS 1 (MIPS 24 μ) | 03 27 39.11 | +30 13 02.8 | 124.7 | 8 |
12 ..... | H2O maser | 03 27 39.10 | +30 13 03.1 | 124.9 | 10 |
13 ..... | HH A | 03 27 38.0 | +30 12 50.5 | 132.8 | 3 |
14 ..... | HH 318 | 03 27 44.8 | +30 14 16.1 | 136.6 | 3 |
15 ..... | L1455 IRS 5 (MIPS 24 μ) | 03 27 38.27 | +30 13 58.5 | 166.3 | 8 |
16 ..... | XMM source 8 | 03 27 38.14 | +30 13 57.7 | 167.0 | 1 |
b) L723 | |||||
1 ....... | XMM source 8 | 19 17 53.04 | +19 12 15.7 | 0.0 | 1 |
2 ....... | 2G N0230300-62415 | 19 17 53.14 | +19 12 16.4 | 1.5 | 11 |
3 ....... | IRAS 19156+1906 | 19 17 53.17 | +19 12 16.5 | 1.9 | 11 |
4 ....... | NH3 W | 19 17 53.02 | +19 12 19.3 | 3.6 | 6 |
5 ....... | CO Peak (red-shifted) | 19 17 53.4 | +19 12 17.1 | 5.2 | 7 |
6 ....... | 350 μ Peak | 19 17 53.41 | +19 12 12.8 | 6.0 | 12 |
7 ....... | VLA 1 | 19 17 52.91 | +19 12 08.9 | 7.0 | 2 |
8 ....... | VLA 2 | 19 17 53.67 | +19 12 19.5 | 9.7 | 2 |
9 ....... | 850 μ Peak | 19 17 53.7 | +19 12 19 | 9.9 | 13 |
10 ..... | H2O maser | 19 17 53.72 | +19 12 19.8 | 10.5 | 6 |
11 ..... | L723 FIR | 19 17 53.9 | +19 12 19 | 12.6 | 4 |
12 ..... | CO Peak (blue-shifted) | 19 17 54.2 | +19 12 10.8 | 16.9 | 7 |
13 ..... | CS Peak | 19 17 54.4 | +19 12 17.2 | 19.3 | 7 |
Notes. Units of right ascension are hours, minutes, and seconds. Units of declination are degrees, arcminutes, and arcseconds. References. (1) This paper; (2) Anglada et al. 1996; (3) Davis et al. 1997a; (4) Davidson 1987; (5) Furuya et al. 2003; (6) Girart et al. 1997; (7) Hirano et al. 1998; (8) Jørgensen et al. 2007; (9) Meehan et al. 1998; (10) Moscadelli et al. 2006; (11) SIMBAD; (12) Wu et al. 2007; (13) Young et al. 2006.
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Two other X-ray sources detected by XMM are plotted in Figure 2. X-ray source 8 (object #16 in Figure 2), a very weak detection, coincides with the heavily obscured Class II YSO IRS 5. IRS 5 is not known to be associated with any molecular outflow, radio jet, or HH structure. X-ray source 9 (object #7 in Figure 2), at best a marginal detection, lies between a pair of nonthermal background radio sources that are unrelated to the L1455 cloud (Schwartz et al. 1985; Meehan et al. 1998). Neither one of the embedded Class I/0 outflow sources in the cloud, IRS 1 or IRS 4, was detected in X-rays. The 3σ upper limit on the count rate signal at the locations of both YSOs in the EPIC pn image is 1.0 count ks−1.
An X-ray light curve and X-ray spectrum were generated for RNO 15 from the filtered EPIC event files using a large circular extraction cell (r = 16'') and offset background region (r = 32''). The broadband count rate of RNO 15 was slightly elevated during the first 4 ks of the exposure (by ∼40%), but was extremely steady during the remainder of the observation, showing no sign of variability on timescales of 600 ks or longer. To analyze the X-ray spectra, I used the XSPEC spectral analysis package from the NASA High Energy Astrophysics Science Archive Research Center (HEASARC) to fit simple multitemperature, optically thin thermal plasma models to the pulse height distribution of the extracted photons. The pn and two MOS spectra were fitted jointly over the 0.9 keV to ∼8.0 keV energy range, varying the hydrogen column density (NH), the source plasma temperature (kT) and metal abundance (Z), and the volume emission measure (EM). Parameters of the optimum model were determined by a χ2 minimization technique. The spectra were binned to a minimum of 15 counts per spectral channel. The latest XMM response files were generated for each camera by means of the SAS arfgen and rmfgen tasks.
The best-fitting model is compared with the observed spectra in Figure 3. The numerical results of the calculation are presented in Table 5. An absorbed one-temperature MEKAL model (Mewe et al. 1995) produced the minimum χ2 and hence the best fit. The temperature of kT = 4.2 keV in this isothermal model is much higher than is typical for a quiescent TTS of Class II (e.g., Getman et al. 2002). Moreover, for a normal gas-to-dust ratio, the hydrogen column density in the model corresponds to an extinction at visible wavelengths of Av ≈ 23 mag (see Bohlin et al. 1978), which is three times the amount suggested by the near-infrared color indices of this star in the color–color plot in Figure 1. A χ2 contour diagram, plotting values of kT against NH (see Figure 4), however, demonstrates that the plasma temperature and column density of the best-fitting model are well constrained by the observation. In addition, no XSPEC model was able to fit the observed spectra with a lower assumed value for the column density; all such models that I tried led to a considerably poorer fit at energies below 2 keV and a much larger value of the χ2. Two-temperature models also offered no significant improvement in the fit. As for the EM and intrinsic X-ray luminosity of RNO 15, the values derived here are at the high end of the range for an active, young TTS, but are by no means exceptional (e.g., Imanishi et al. 2003; Gagné et al. 2004).
Figure 3. Top: X-ray spectrum of RNO 15 (XMM source 10) observed with the EPIC cameras on XMM. A single-temperature thermal model computed with the XSPEC MEKAL plasma emission code and folded through the XMM detector response is plotted as a histogram curve. Channels in the observed spectra have been merged to yield a minimum of 15 counts per bin. The upper curve is the pulse-height spectrum from the pn camera. The two lower curves are from the MOS cameras. The narrow emission feature just below 7 keV can be identified with a blend of inner shell lines of helium-like Fe xxv at 6.7 keV and hydrogen-like Fe xxvi at 6.9 keV. Bottom: The contributions of each bin to the value of χ2. A positive (negative) value indicates that the observed point lies above (below) the fitted model.
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Standard image High-resolution imageFigure 4. Plasma temperature vs. hydrogen column density χ2 contours for the XSPEC MEKAL model of RNO 15. Contours are shown for 1σ, 90%, and 99% confidence.
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Standard image High-resolution imageTable 5. XSPEC Model X–Ray Flux and Luminosity for RNO 15
NH (1022 cm−2) | kT (keV) | Z/Z☉ | log EM (cm−3) | χ2ν/d.o.f.a | fXb (10−12 erg cm−2 s−1) | log LXc (erg s−1) |
---|---|---|---|---|---|---|
4.47 ± 0.31 | 4.24 ± 0.61 | 0.19 ± 0.06 | 54.02 ± 0.08 | 1.06/87 | 1.21 ± 0.12 | 31.17 ± 0.08 |
Notes. Quoted errors are 1σ and were determined using the XSPEC error command. aReduced χ2 and degrees of freedom for XSPEC model. bAbsorption-corrected flux at Earth in the 0.3–10.0 keV energy band. cAbsorption-corrected 0.3–10.0 keV X-ray luminosity for a distance of 320 ± 30 pc (Hatchell et al. 2007; Wu et al. 2004).
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The remaining X-ray sources listed in Table 2 are much fainter than RNO 15, consequently none of them could be modeled in the same fashion as that star. I obtained a crude estimate for the X-ray luminosity of the other embedded YSO of obvious interest, XMM source 8, by matching its observed count rate with a one-component MEKAL model. I assumed a value of 2 keV for the unknown plasma temperature and adopted a NH value appropriate to the extinction of Av = 30 mag that was derived earlier from the near-infrared colors of its infrared counterpart IRS 5. Given those assumed values, the observed count rate yields an extremely tentative estimate of LX ∼ 1030.4 erg s−1 for the X-ray luminosity.
3.2. The L723 Cloud
The center of the XMM field of view is shown in the IRAC 4.5 μ image of L723 in Figure 5. Table 4 provides a key to the symbols plotted there. The cross labeled as object #1 corresponds to X-ray source 8. The uncertainty in the right ascension and declination of the X-ray centroid determined by the SAS edetect task for that source is ±25 at the 95% confidence level. The nominal position of IRAS 19156+1906 lies 2'' east of the X-ray position and has an uncertainty in right ascension of ±12'' (Moshir et al. 1990). Hence, the X-ray centroid falls within the error ellipse of the IRAS source. The two objects are the likely shorter and longer wavelength counterparts, respectively, of the faint Hubble Space Telescope (HST) guide star and its 2MASS and anonymous near-infrared complement that is found at the same location in the IRAC image.
Figure 5. Gray-scale Spitzer IRAC 4.5 μ image of the central portion of the L723 dark cloud. The objects are identified by name in Table 4. IRAS 19156+1906 (object #3) lies slightly above and to the right of center in the field of view, VLA 2 (object #8) slightly above and to the left of center. The plus symbol is XMM X-ray source number 8, and the crosses denote named radio, submillimeter, and optical sources.
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Standard image High-resolution imageVirtually nothing is known about the optical guide star according to the SIMBAD database. As I noted in Section 2.3, the 2MASS colors of the star are compatible with those of an unreddened or lightly reddened normal field giant. However, if that were so, the 2MASS magnitudes of the star would place it at a distance of >16 kpc and consequently too far away to have been detected as a background X-ray source. The more reasonable alternative is that the star lies within the L723 cloud and is a marginal or weak-line TTS with a very small infrared excess and at most a slight amount of reddening. The X-ray source itself is too faint to be reliably modeled using XSPEC. A rough guess at the X-ray luminosity of the star can be deduced from the observed count rate using a very simple one-component MEKAL model. Assuming a nominal extinction of Av = 2 mag, a plasma temperature kT in the range of 1–4 keV, and a distance of 300 pc for L723 (Goldsmith et al. 1984), I derive a luminosity of LX ≈ 1029.3 erg s−1. Although highly uncertain, that LX is very typical of the X-ray luminosities of weak-line TTSs (e.g., Neuhäuser 1997). Nearly the same value, LX ≈ 1029.36 erg s−1, was found for KP2-44, the borderline TTS that powers the HH 189 jet in L1251 (Simon 2006), and which shares the same infrared colors as the source in L723.
Several other noteworthy objects in the XMM field of view toward L723 deserve mention here. VLA 2 is the double radio source whose binary components have been proposed as the separate origin and driving source of the two high-velocity flows in L723 (Carrasco-González et al. 2008). No X-ray emission was detected at their location. The 3σ upper limit on possible emission at the radio position in the pn camera is 1.84 count ks−1. VLA 1, a second radio source located 7'' south of the X-ray source, was discovered at the same time as VLA 2. It was initially proposed as the possible launch site for the narrow north–south flow in L723 (Anglada et al. 1991), but was subsequently dismissed as a likely background source (Anglada et al. 1996). No X-ray emission was observed at the position of VLA 1. Source number 1 (XMMU J191651.1+191520), the brightest X-ray source detected by XMM in the L723 exposure, was outside the MOS field of view and very close to the outer edge of the pn image. The X-ray SED and 2MASS photometry are consistent with a one-component MEKAL model in which Av = 2.8 mag and kT = 1.4 keV, the distance is d = 125 pc, and the spectral type is mid-G. I infer a source luminosity of LX = 1029.92 erg s−1, which suggests a very active field star, possibly of the RS CVn variety.
4. CONCLUSIONS
L1455 and L723 are dark clouds that are known for the complex morphology of their molecular flows. The spatial distribution of their high-velocity CO gas evinces a quadrupolar geometry, which suggests the obvious physical model of several independent, overlapping bipolar flows that emerge from different locations within the cloud. The same pattern of velocities is susceptible, however, to a different interpretation that calls for a single high-speed wind with a very large opening angle. A major goal of the present study was to see whether the ambiguity might be resolved by X-ray observations of the YSOs that are believed to be the driving sources of the flows.
In L1455, X-ray emission was detected from a pair of Class II YSOs, RNO 15 and L1455 IRS 5. Neither star is known to be associated with an outflow in the cloud. On the other hand, neither of the two Class I/0 stars thought to drive an outflow, IRS 1 and IRS 4, was detected in X rays. If the very similar L1251 cloud is a suitable example, the quiescent X-ray emission of the deeply embedded Class I/0 YSO sources in L1455 may be too faint to be seen above the high instrumental background of the EPIC cameras aboard XMM. To detect such sources may require the lower background levels of the ACIS camera on Chandra.
The situation in L723 presents a similar picture. No X-ray emission was observed at the position of VLA 2, the continuum radio source that is thought to be the origin of the strong east–west outflow in the cloud. Weak emission was detected, however, at a nearby position that has been identified as the possible source of the more tightly collimated north–south flow. The coordinates of the X-ray source coincide with those of an NH3 condensation (Girart et al. 1997), the nominal position of a Class 0 IRAS source, and a near-infrared 2MASS source with a faint optical counterpart. There is a considerable resemblance between the L723 source and KP2-44, a TTS in the L1251 cloud that powers the HH 189 jet. However, there is also a potentially significant difference as well: the YSO in L723 is associated with no known HH object (Palacios & Eiroa 1999; López et al. 2006), whereas KP2-44 is associated with no known CO outflow.
This research has made use of SAOImage ds9 and the Funtools software, developed by Smithsonian Astrophysical Observatory, as well as the Aladin Sky Atlas and SIMBAD database, maintained by CDS, Strasbourg, France. It is based on observations made with the XMM-Newton Observatory, an ESA science mission with instruments and contributions funded by ESA member states and the National Aeronautics and Space Administration (NASA). Support for this work was provided by NASA through XMM Guest Observer award number NNG07EB02P, which is gratefully acknowledged. The DSS were produced at the STScI under NASA Grant NAG W-2166 and are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope with the permission of those institutions. The Guide Star Catalog-II is a joint project of STScI and the Osservatorio Astronomico di Torino. STScI is operated by the Association of Universities for Research in Astronomy, for NASA under contract NAS5-26555. The participation of the Osservatorio Astronomico di Torino is supported by the Italian Council for Research in Astronomy. Additional support is provided by European Southern Observatory, Space Telescope European Coordinating Facility, the International GEMINI project and the European Space Agency Astrophysics Division. The 2MASS is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC)/California Institute of Technology, funded by NASA and the National Science Foundation.
Facilities: XMM (EPIC).
Footnotes
- 1
An updated version can be found at http://heasarc.gsfc.nasa.gov/docs/xmm/abc/.