[vsnet-alert 22688] Gaia18dcu (ATEL)

[Taichi Kato]

Gaia18dcu (ATEL)

Gaia:
194124.045 +393141.24 (2000.0) Gaia_DR2_2076265604114635648 plx=1.277(0.991) dismod=9.5 pmra=-0.571(2.467) pmdec=-24.448(3.013) G=20.457 BP=20.173 RP=19.622

   High-proper motion object.  The parallax tells almost nothing
about the distance.

===

ATEL #12173                                                          ATEL #12173

Title:  Gaia18dcu: a three-magnitude brightness increase from a 50,000K
                stellar object
Author: B. Mccollum (American Univ.), S. Laine (Caltech/IPAC), F.
                C. Bruhweiler (American Univ.)
Queries:        mccollum at american.edu
Posted: 6 Nov 2018; 00:58 UT
Subjects:Infra-Red, Optical, Ultra-Violet, A Comment, Transient, Variables


We call attention to the unusual host object of a blue transient, Gaia18dcu,
in the Kepler field.  

The object is noteworthy in having a very hot quiescent SED which we find
dominates not only the UV but also the visible bandpasses, a quiescent
Lbol of at least ~11x Lsun over several years, and a strong H-alpha excess
in at least one quiescent epoch.  

Gaia reports that it has shown a historical variability of ~0.5 magnitude
and a G magnitude of ~20.5.  It showed an increase of ~3.5 magnitudes in
the outburst detected by Gaia.  Variability of ~0.5 magnitude is also seen
in two catalog r band values. 

We performed SED fitting using catalog data.  We used data from the Pan-STARRS
Release 1, the Kepler INT Survey (Greiss et al. 2012, AJ, 144, 24), Gaia
catalog photometry, and the GALEX NUV Survey (Olmedo et al. 2015, ApJ,
813, 100).   It was detected by NEOWISE-R.  We obtained additional measurements
from archival GALEX FUV and Spitzer IRAC images.    

We fitted the visible-UV SED to a single-object blackbody.  The Av was
allowed to vary as a free parameter from 0.30 to 10, with the lower limit
set by the 3-D extinction map of Green et al. (2018; MNRAS, 478, 651).
The Teff was varied from 1000 K to 100,000 K in 100 K increments.  Only
a very poor fit could be obtained for the combined UV-visible-IR SED, but
a good fit using UV and optical data, so we excluded IR data from the fitting
except as upper limits.  Comparison with the blackbody SED suggests an
IR excess.  Also, there is evidence of variability by a factor of several
in across two epochs in the Spitzer and WISE IR photometry. 

The best fit was obtained for a blackbody of 50,000 +/- 2000 K with Av
= 1.81 +/- 024.  The fact that the Av value is much larger than expected
from interstellar extinction supports the presence of an IR excess from
dust.  

The SED and best-fit blackbody may be viewed at  

http://spider.ipac.caltech.edu/staff/seppo/gaia-figures/Gaia18dcu_SED.png

and 

http://spider.ipac.caltech.edu/staff/seppo/gaia-figures/Gaia18dcu_SED_fit.png

The nearest X-ray or gamma ray source cataloged is 12.8 arcmin +/- 15 arcsec
distant.  Nondetection by the RASS survey places a very approximate X-ray
upper limit of ~10x the GALEX FUV flux, consistent with a ~50,000 K blackbody.

Based on the blackbody SED, we estimate a bolometric luminosity of 11x
Lsun to 85x  Lsun, where the uncertainty is reflects the Gaia distance
uncertainty (1289 to 7355 pc). 

The Lbol is a few orders of magnitude less than expected of a 50,000 K
stellar source. This along with the H-alpha excess suggests that the optical-UV
emission is dominated by an accretion disk.   

Gaia18dcu is not in the 2MASS catalog, but from the catalog image we placed
an upper limit of  Ks ~18 +/- 0.5.  This is roughly two magnitudes fainter
than the absolute magnitude of a main sequence late M star at ~1200 pc.
Thus any cool object in the system is probably not earlier than spectral
type M unless it is well beyond the Gaia ~7300 pc one-sigma maximum distance.

We suggest that Gaia18dcu might be a previously unknown interacting binary.
Spectroscopy of this object is encouraged.   

This research has used the VizieR catalogue access too, CDS, Strasbourg,
France.  We acknowledge the Pan-STARRS1 Survey (PS1) and the PS1 public
science archive. This publication makes use of VOSA, developed under the
Spanish Virtual Observatory project supported from the Spanish MINECO through
grant AyA2017-84089. This publication makes use of data products from the
Two Micron All Sky Survey. We acknowledge ESA Gaia, DPAC and the Photometric
Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts).  This publication
makes use of data products from the Near-Earth Object Wide-field Infrared
Survey Explorer (NEOWISE), which is a project of JPL/Caltech.