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Gaia16aau is a new R Coronae Borealis star in the SMC

ATel #8681; P. Tisserand (IAP), L. Wyrzykowski (Warsaw University Astronomical Observatory), G. Clayton (Louisiana State University), D. Welch (McMaster University), A. Udalski (Warsaw University Astronomical Observatory)
on 11 Feb 2016; 21:52 UT
Credential Certification: Lukasz Wyrzykowski (wyrzykow@astrouw.edu.pl)

Subjects: Optical, Transient

Gaia16aau has been detected by the Gaia Science Alerts system (http://gsaweb.ast.cam.ac.uk/alerts) in data collected by the Gaia space mission on Jan.25,2016 and published on Jan.30, 2016. It has shown a very large amplitude change on a supergiant star, named [MH95] 580, located in the Small Magellanic Cloud (SMC), resembling a slow recovery phase since January 2015, from 19th to the 15th magnitude.

The object happened to be located within the fields of the OGLE-IV survey (Udalski et al. 2015) where the star has shown a previous sudden decline of about 7 mag at the beginning of March 2012, after having remained bright at I~13.6 mag for more than 2 years before that (see OGLE-IV lightcurve of OGLE-SMC710.08.1 on RCOM: http://ogle.astrouw.edu.pl/ogle4/rcom/rcom.html, Udalski,2008).

[MH95] 580 was catalogued as a carbon star by Morgan and Hatzidimitriou in 1995 after an investigation of UK Schmidt objective-prism plates. Using the Infrared selection technique described in Tisserand (2012), this star was on a short list of targets of interest rich in R Coronae Borealis stars (RCBs) using the 2MASS and the WISE All-Sky surveys. Indeed, these rare and interesting stars are producing dust that creates warm circumstellar shells. Their resulting spectral energy distribution made of two black bodies (a F/G star and a warm shell, 600<T_shell<900 K) is very specific and they can therefore be discriminated from other dust-producing stars. [MH95] 580 is listed as object #5003 in the new targets of interest list that will be published shortly by Tisserand, Clayton and Welch (2016). This RCB star candidate was followed-up on the 24th and 26th July 2013 using the WiFes spectrograph mounted on the 2.3m telescope located at Siding Spring Observatory (Australia). At that time, [MH95] 580 was still in its recovery phase, at I~15.2 mag. The mid-resolution spectrum (R~3000) shows no CN band heads but clear C_2 bandheads below 5800 angs, as well a many absorption lines that are typical of warm RCB stars, like C I, N I, O I and Fe lines, as well as a weak H-alpha absorption line, which is rarely seen in RCB stars. From the strength of the C_2 features and synthetic spectra of Hydrogen deficient and carbon rich stars, we can estimate an effective temperature between 6700 and 7000 K.

Gaia16aau, also named [MH95] 580 and OGLE-SMC710.08.1, is a new R Coronae Borealis-type star, the 5th known in the SMC. The photometric behaviour observed by Gaia is a characteristic signature of RCB-type stars. These unpredictable rapid fadings are thought to be due to the photosphere being obscured by newly formed dust clouds along the line of sight. As the dust clouds disperse, the star gradually returns to its original brightness over a period of months.

RCBs are rare as only a hundred are actually known. They are the best known candidates to be the products of white dwarf mergers in the intermediate mass range (0.4<Mtot<1.1 M_Sun). After a merger, a He envelope surrounds a CO core, and a short-lived supergiant phase starts. RCBs correspond to that phase.

The Gaia Science Alerts system has demonstrated with this first trigger on an RCB-type star that it can discover many more of them in our Galaxy and the Magellanic Clouds. Depending on the models, we can expect that between 100 and 5000 RCB stars exist in our Galaxy.

We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts).

OGLE real-time light curve is available here: