Strong near-IR fading observed in the young source MJS2008 133 in Lupus

ATel #4712; Simone Antoniucci, Dario Lorenzetti, Brunella Nisini, Teresa Giannini (INAF-Osservatorio Astronomico di Roma)
on 7 Jan 2013; 16:45 UT
Credential Certification: Simone Antoniucci (antoniucci@oa-roma.inaf.it)

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We report that the young stellar object MJS2008 133 (coord. J2000 16 12 11.200 -38 32 19.74) underwent a fading of about 2 magnitudes in the near-IR H band (1.65 μm). Up to now, this source had not been classified as a variable. The discovery was done while analysing H-band photometric data of the POISSON survey (Antoniucci et al. 2011, A&A 534, A32), which collected near-IR spectra (JHK, 1-2.5 μm) of selected pre-main sequence targets in various star-forming regions.
The data were acquired on July 2009 with NTT/SofI at ESO La Silla. We measure a H magnitude of 12.19 ± 0.05, to be compared with the previous 2MASS (Skrutskie et al. 2006, AJ, 131, 1163) H-band value of 10.278 ± 0.023 mag, obtained in June 2000.
From the spectrum of the source, we were also able to derive an estimate of the magnitude in the other near-IR bands. These values can be compared to J and K magnitudes from both 2MASS and DENIS (Epchtein et al., 1997 Msngr, 87, 27). All the near-IR photometric measurements obtained on the source are summarised in the following table.

Survey	date	J	H	K
DENIS	Jun 1999	11.90±0.07	--	9.56±0.06
2MASS	Jun 2000	11.841±0.024	10.278±0.023	9.406±0.022
POISSON	Jul 2009	13.3±0.1	12.19±0.05	11.4±0.1

The magnitudes of the source did not significantly vary between DENIS and 2MASS observations (i.e. for a time interval of about about one year). More interestingly, both the object near-IR colours (J-H) and (H-K) are smaller with respect to 2MASS, so that the source now appears bluer than in its brighter state. This colour change is opposite to the one expected for either an outburst event connected to an episode of increased mass accretion (as in EXor sources, see e.g. Lorenzetti et al. 2012 ApJ, 749, 188) and a variation of the extinction along the line of sight. In both cases, sources become bluer (redder) as their luminosity increases (decreases).
The same type of variability (with a bluer faint state) has been already observed in a few young sources by Carpenter et al. (2001 AJ, 121, 3160), who suggest this may come from a variation of the inner radius of the circumstellar disc. If the disc radius increases the system becomes fainter in the near-IR and bluer, with less emission absorbed and reprocessed by the disc.
An estimate of the mid-IR variability of MJS2008 133 may be obtained from Spitzer (Evans et al. 2003, PASP, 115, 965) and WISE (Wright et al., 2010 AJ, 140, 1868) observations, taken in 2004 and 2010, respectively. A meaningful comparison can be made in the WISE W1 (3.6 μm), W2 (4.6 μm), and W4 (22 μm) bands, which well match the Spitzer band-passes (at 3.6, 4.6, 22 μm). The differences [mag (WISE) - mag(Spitzer)] are 0.5, 0.1, 0.1 (at 3.6, 4.6, and 22 μm, respectively). Given the uncertainties of both observations, we can conclude that a marginal indication of mid-IR variability is present only at the shortest wavelength. Alternately, this might suggest that the main brightness variation occurred sometime between 2000 (2MASS) and 2004 (Spitzer).
Further observations are warmly encouraged.