ASASSN-18dw - Constraints on the Progenitor from Near-Infrared Archival Photometry

ATel #11362; Peter Pessev (GRANTECAN, IAC, ULL)
on 27 Feb 2018; 15:07 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Peter Pessev (peter.pessev@gtc.iac.es)

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ASASSN-18dw was discovered on UT 2018-02-25.14 at g~14.3 as a transient with an amplitude of more than four magnitudes by Stanek et al. (ATel#11359). An analysis of the available archive near-IR photometry is presented in an attempt to constrain the properties of the progenitor.

A search in the Two Micron All Sky Syrvey (2MASS; Skrutskie et al. 2006 AJ 131, 1163) and The Deep Near Infrared Southern Sky Survey (DENIS; Epchtein et al. 1997 Msngr 87, 27) databases revealed source counterparts, which properties are summarized below:

2MASS
Epoch of observation (JD) = 2451206.5658
J = 15.686+/-0.073
H = 15.117+/-0.083
Ks = 14.902+/-0.140

DENIS
Epoch of observation (JD) = 2451514.751759
I = 16.86+/-0.10
J = 15.72+/-0.20

In the case of 2MASS All-Sky Release Point Source Catalog the counterpart was located 0.16 arcseconds away from the position reported by Stanek et al. 2018 (ATel#11359) and the DENIS DR3 (September 2005) position is 0.12 arcseconds away. Looking at the J band magnitudes of the source, it is obvious that the source did not show significant variations in brightness between Jan. 28th 1999 and Dec. 2nd 1999. Unfortunately both surveys were conducted simultaneously (1997-2001 for 2MASS versus 1996-2001 for DENIS) and do not give the chance to establish longer variability baseline. Also taking into account that the source was detected only in the I and J bands of DENIS, it is not possible to apply the transformations of Carpenter 2001 (AJ 121, 2851). On the other hand, considering the stable J magnitudes it is possible to combine both datasets and compute colors that will set some limits on the nature of the progenitor.

The Galactic Dust Reddening and Extinction service within IRSA gives the following values for the dust extinction in a 2 degrees region around the transient position A_I=0.444, A_J=0.212, A_H=0.135, A_Ks=0.091 and A_V=0.786 magnitudes for the Schlafly & Finkbeiner 2011 (ApJ 737, 103) reddening maps. The corresponding colors, corrected for extinction are:

(I-J)_0 = 0.946+/-0.124
(J-H)_0 = 0.492+/-0.111
(H-Ks)_0 = 0.171+/-0.163

Please note that only the errors of the photometry are taken into account when calculating the uncertainties of the colors. Taking into account the uncertainties, these colors are consistent with a main sequence star of K spectral class, according to the data presented by Pecaut & Mamajek 2013 (ApJS 208, 9). They are also consistent with the same spectral and luminosity class according to the data presented in Ducati et al. 2001 (ApJ 558, 309), although some systematic differences in the (I-J) color are present in this case. Only based on the colors, it is not possible to rule out that the progenitor of the outburst does not belong to the giant or supergiant luminosity class.

In case of a K main sequence star, utilizing the data of Pecaut & Mamajek 2013 (ApJS 208, 9), a distance modulus between 9.8 and 10.1 could be estimated, which means that the object is located further than the the Orion star forming region. In the case of more luminous object (giant or supergiant), the distance will only increase. Although there is a significant number of heterogeneous photometric data points on the progenitor in different archives, follow up photometry and spectroscopy are crucial to better constrain the properties of the transient.