The Weakening Outburst of the Young Eruptive Star V582 Aur

Our new photometric results are plotted in Figure 1. For reference we also plotted the earlier light curves from A18 until 2018 January 7. Following the minimum of 2017 February, the optical light curves show a brightening trend until 2018 February. In A18, we expected that this trend would continue and the source would reach the maximum brightness level (measured in 2013–2014, see horizontal lines in Figure 1) during 2018. However, our new optical data indicate a break in this trend. After reaching a peak in 2018 February, the source started dimming again. The latest observations from 2018 August–2019 February show similar brightness levels to the beginning of the year, interrupted by a local minimum in 2018 October. Currently the source is as bright as it was in 2018 August–September, but is still about 1 mag fainter than it was in 2013–2014.

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After 2018 February, the shapes of the B, V, R_C, and I_C light curves are more similar to each other than in 2016/2017 (Figure 1). Our near-infrared H band photometric point on 2018 April 15 shows the same brightness level as the last near-infrared data point in 2017 April. On 2018 November 10 the source was significantly brighter in the J and H bands than in 2017, while no change in its K_S brightness was detected. These magnitudes are, however, still fainter by 0.5–0.7 mag than the maximum brightness levels before 2016 (see A18). Our photometric results imply that the dimming of V582 Aur, which started in 2016, has not finished yet, and its length is at least 3 yr, significantly exceeding the length of the 2012 dimming.

In Figure 2, we show the color–magnitude diagrams based on our optical and near-infrared data. We plotted data points measured before 2014 (black circles), photometry from 2014 to 2017 (blue circles), and our new observations from Table 1 (filled red circles). The earlier data points did not populate the brightness range between V = 15.5 and 16.5 mag. The new data points after 2018 January fill this brightness gap, representing an intermediate brightness level. Figures 2(a)–(c) suggest that the data points obtained before 2014 and the data points measured in 2014–2017 occupy different locations in the color–magnitude space. Comparing the colors of the two samples at any given V band magnitude, the earlier data (black circles) are bluer in Figure 2(a), have similar [V − R] colors in Figure 2(b), and are systematically redder in Figure 2(c) than the 2014–2017 measurements. Moreover, there is an additional discrepancy in the distribution of points before and after 2014. They exhibit similar but parallel patterns in such a way that the data after 2014 seem to be ∼0.3 mag dimmer in V band for the same color. In Figures 2(a)–(c), our new photometry after 2018 January remarkably match the color–magnitude trends outlined by the data points from 2014 to 2017.

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Figure 2(d), which shows an I_C versus [I_C − H] diagram, indicates that the new data points seem to match the trend outlined by the 2014–2017 measurements. Extrapolating from the 2014–2017 data to brighter I_C magnitudes, there is a hint that the data points before 2014 were systematically redder at a given I-band magnitude than the later photometry, similarly to Figure 2(c).

All available WISE measurements from Table 2 are plotted in Figure 3. The first two measurements during 2010 revealed a slight brightening of the source. By the beginning of 2014, V582 Aur was fainter again, and the drop of brightness with respect to 2010 was significantly larger in the W2 than in the W1 band. During 2014, the WISE light curves show a brightening trend, and after reaching a maximum in 2015 March both light curves started a linear fading phase, which is still followed by the source.

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In A18, we demonstrated that the optical and near-infrared variability of V582 Aur could be, to a large part explained by changing extinction in the line of sight. In order to check if the midinfrared variability, measured by WISE, was also due to extinction variations, we compared the midinfrared light curves with the optical ones. We overplotted in Figure 3 a modified version of the I_C light curve scaled down by the relative amplitudes of the interstellar extinction at the respective wavelengths (0.79 and 3.4 μm). This scaled down light curve would be the expected midinfrared behavior if the light variations were caused by the same extinction at both the optical and near-infrared wavelengths. Interestingly, the measured WISE light curves do not follow the expected trend, suggesting a different physical mechanism as the origin of the variation. The [W1−W2] color variations during the observed period, plotted in the bottom panel of Figure 3, show that the source was bluer in early 2014 and early 2016, while it was redder in 2017, although not as red as in 2010.

In A18, following Kun et al. (2017), we assumed a distance of 1.32 kpc for V582 Aur. The Gaia DR2 parallax, however, implies a larger distance of ${2.4}_{-0.4}^{+0.7}$ kpc (Bailer-Jones et al. 2018). Note that V582 Aur seems to be associated with a complex of bright rim clouds (see Figure 3 in Kun et al. 2017). The closest OB star, thus the most likely ionization source is HD 281147, whose Gaia DR2 distance is ${2.4}_{-0.2}^{+0.3}$ kpc. This result also supports the new distance value for V582 Aur. We also checked the Gaia DR2 distances for a list of stars used to define the Auriga OB1 and Auriga OB2 associations by Humphreys (1978). We found that the distances for the two associations overlap, although Aur OB1 (d = 1.9 ± 0.7 kpc) seems to be closer to the Sun than Aur OB2 (d = 2.8 ± 0.5 kpc). Based on these results, it is unclear which association V582 Aur belongs to. Adopting the distance of 2.4 kpc for the FUor, several fundamental parameters would change. The luminosity and accretion rate would increase to 500–1050 L_⊙ and 8 × 10⁻⁵ M_⊙ yr⁻¹ respectively, making the object perhaps the most luminous FUor in the list of Audard et al. (2014). The mass of the circumstellar material derived in A18 would also change to 0.1 M_⊙.

In A18, we demonstrated that the multiwavelength brightness variations of V582 Aur could be explained by variable extinction toward the source. We explored two possibilities depending on whether the eclipses in 2012 and in 2016–2018 were caused by the same orbiting dust clump, or by two independent dust structures. Based on the first scenario, we speculated about an elongated dust clump orbiting the star at a radius of 2.8 au, having a length of ∼3.5 au, and a mass of 0.004 M_⊕. Our new results show that the variability detected after 2018 February also matches the previous color–magnitude trends (Figure 2), implying that changing extinction is still the dominant physical mechanism behind the light variations. However, just this fact does not help us decide between the two possibilities. In the following, we discuss how our observations are consistent with one or the other scenario.

In the light of our new data, the 2016–2018 minimum significantly differs in length and shape from the one in 2012. Therefore, if both fading events were caused by the same dust clump, as we supposed in A18, then the clump covers a more extended part of the orbit in 2016–2018 than in 2012. Since the length of the current minimum is at least 2 yr, the clump could be stretched over ∼7 au if adopting an orbital radius of 2.8 au from A18. It suggests a viscous spreading of the dust particles along the orbit. Since the WISE measurements do not follow the scaled down I_C-band light curve (Figure 3), either the orbiting dust clump does not cover that part of the inner disk where the midinfrared radiation is emitted or the clump itself contributes to the midinfrared radiation.

The other explanation for the observed dimming events includes individual dust clouds entering and leaving the inner part of the system as giant exocomets. In this picture the inward moving dust structure might pass the line of sight and may be responsible for the optical dimming events. Approaching the central object, the dust warms up, increasing the midinfrared thermal emission of the system and making the observed [W1–W2] WISE color bluer. We speculate that this may be the reason of the midinfrared brightening of V582 Aur in 2014 and early 2015 (Figure 3). When the dust structure is leaving the system, it cools down and emits less at the midinfrared WISE wavelengths. We may see the beginning of this effect in the latest WISE measurements.

The light curves and the color evolution of V582 Aur show striking similarity to those of another well known young stellar object, RW Aur A (Dodin et al. 2019, and the references therein). The physical reason for the behavior of RW Aur A is still debated. The possible mechanisms, like dusty disk wind (Petrov et al. 2015; Bozhinova et al. 2016), or a warped inner disk (Facchini et al. 2016) might be invoked to explain the variability of V582 Aur as well.

The color–magnitude diagram (Figure 2) indicates that the distribution of points before and after 2014 outlined similar patterns; however, the later data set has ∼0.3 mag fainter values in the V-band. In A18, we identified the origin of the pattern as a combination of the interstellar extinction and the UXor blueing effect. UXors, a subclass of Herbig Ae/Be stars named after the prototype object UX Orionis, are defined by their irregular deep brightness minima in the light curve. As they start fading, their colors become redder in the color–magnitude diagram; however, after reaching a certain faintness, they exhibit a color reversal. This is the so-called blueing effect (The 1994; Natta & Whitney 2000, and references therein). The most widely accepted physical explanation for this phenomenon assumes a system seen almost edge-on. As a dust clump begins obscuring the star, the observed light becomes redder according to the dust extinction law. When the star is completely obscured by the dust clump, only the bluer scattered light from the circumstellar environment remains, causing a color reversal in the color–magnitude diagram. In Figure 2, the upper linear part of the pattern is parallel to the extinction path shown by lines in the panels. The lower horizontal part is probably the signature that the central object is significantly obscured and the remaining emission is dominated by scattered light. Here, we speculate that the ∼0.3 mag difference arises from the absolute fading of the source, reflecting the long-term evolution of the outburst. In this scenario, the 0.3 mag fading during ∼5 yr would mean that the source will reach the quiescent level in approximately 80 yr. This decades long trend is characteristic for FUor type stars. Future observations will show whether the experienced color change continues. The light curves of the WISE measurements differs significantly from those at the optical wavelengths, suggesting different underlying phenomena.

In A18, we fitted the near-infrared spectral energy distributions at different epochs with a simple accretion disk model. We adopted a steady, optically thick, and geometrically thin viscous accretion disk, with a radially constant mass accretion rate. We found that until 2017 April, the mass accretion rate was approximately constant, while the line-of-sight extinction episodically increased during the dimming periods in 2012 and in 2016–2018. Here, we repeat the modeling using the 2.4 kpc distance measured by Gaia, and also including our new J, H, and K_S measurements from 2018 November 10 and the 2MASS measurements from 1998 October 8. Our results are plotted on Figure 4. The comparison of this figure and Figure 11 in A18 shows that the model provided higher accretion rates due to the larger distance of 2.4 kpc. The line-of-sight extinction also increased in the new calculations, since the higher accretion rate corresponds to hotter disks. This requires higher line-of-sight extinction in order to reproduce our measurements. For the new epoch in 2018 we derived an extinction of A_V ≃ 6.5 mag, which is significantly lower than what we obtained for the deepest minimum in 2017 February and is only slightly higher than the values measured outside the dimmings, in the maximum brightness state. The corresponding accretion rate value in 2018 November, however, is lower by a factor of 2 than was in the maximum brightness state before 2017. Comparing these results with earlier ones, we found that the extinction in 1998 was on the same level as in 2010–2013, apart from the dimming in 2012. The accretion rate, however, was then higher than at any time later. The long-term decreasing trend in the accretion rate is consistent with our previous conclusion on the long-term fading of the source (Section 4.2).

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Our results indicate that the light curves of V582 Aur show the combined effect of extinction related dimming events and a general long-term decay of the outburst. The latter trend predicts that the source will never return to the pre-2014 state and the current brightness levels may be already close to an unobscured state with a lower central luminosity. Future observations can verify this conclusion and constrain the full length of the eruption. For instance, the ongoing Gaia sky survey will provide improved photometry in two optical passbands starting from 2014. Using the Gaia G_RP band observations, which is close to the I_C band (see Figure 3 in Jordi et al. 2010), we will be able to not only compare our ground-based measurements with the high precision Gaia photometric results, but we can also examine the long-term variability of V582 Aur. During the nominal 5 yr mission, Gaia will provide approximately 50 photometric points for V582 Aur, which will cover a large part or the whole ongoing dimming event. Furthermore, the Transiting Exoplanet Survey Satellite (TESS) will also carry out observations in the direction of Auriga, providing 30 minute cadence photometry in a broad band, which is centered on the effective wavelength of the I_C band (see Figure 1 in Ricker et al. 2015). Considering V582 Aur's ecliptic latitude, TESS will probably provide a 28 day long uninterrupted light curve for this target. These high cadence measurements can reveal the short-term or small amplitude variations, which are invisible for ground-based telescopes and may reveal the fine structure of the obscuring dust cloud.