Unusual variable in Sagittarius (ASAS J180645-3604.9)

This report in vsnet-alert 14824 is extremely interesting.
A star that slowly brightened from who knows what magnitude to mag. 12 in 2005 and in late 2008 started to show large amplitude rapid variations.
I copy the vsnet-alert messages below so you know what it's about.

Spectroscopic and photometric observations are welcome.
Beware of the 14th mag. close companion!

Cheers,
Sebastian
 

----- Original Message -----
From: "Hiroyuki Maehara" <maehara@kwasan.kyoto-u.ac.jp>
To: <vsnet-alert@ooruri.kusastro.kyoto-u.ac.jp>; <vsnet-newvar@ooruri.kusastro.kyoto-u.ac.jp>
Sent: Friday, August 03, 2012 11:24 PM
Subject: [vsnet-alert 14824] PNV_J180645.2-360458

Shigehisa Fujikawa (Kagawa, Japan) reported his discovery of an unusual
variable star (or possible nova?) in Sgr. His observations and a follow-up
observation by K. Itagaki (Yamagata, Japan) are as follows (measured by
H. Kaneda, Hokkaido, Japan):

2012 Jul 23.517     12.5  (S. Fujikawa; f=105mm lens + CCD)
           23.519     12.6  (S. Fujikawa; f=105mm lens + CCD)
           30.554     12.2  (S. Fujikawa; f=105mm lens + CCD)
           30.555     12.0  (S. Fujikawa; f=105mm lens + CCD)
   Aug  1.553       11.5  (S. Fujikawa; f=105mm lens + CCD)
           1.605       11.3  (S. Fujikawa; f=400mm lens + Canon EOS X2 DSLR camera)
           2.467       11.3  (K. Itagaki; 60cm telescope + CCD)
           2.491       11.5  (S. Fujikawa; f=400mm lens + Canon EOS X2 DSLR camera)

 The object is located at R.A. =  18h 06m 45.23s, Decl. = -36d 04' 58.5"
(equinox 2000.0; measured from 6 CCD images obtained by K. Itagaki with
60cm telescope). There is a faint object in the 2MASS catalog
(2MASS J18064523-3604583 Jmag=15.11) at this position.

 H. Kaneda pointed that this object was detected by ASAS. According to
ASAS data, this object brightened slowly during 2003-2005 and it was
at almost constant brightness (V~12mag) during 2005-2008. The object
showed short fadings (time-scale of a few tens of days) in the 2009
observing season.
http://www.astrouw.edu.pl/cgi-asas/asas_variable/180645-3604.9,asas3,0,…
Note: there is a mag~13-14 star at 18:06:45.67, -36:04:55.1 (J2000.0),
about 6 arcseconds NE of this object.

The long-term variation of the object is similar to those of V4334 Sgr
and FG Sge (or very slow nova?). Could someone perform spectroscopy of
the object? This object was at Vmag=11.2 (B-V=1.1) on Aug. 3.50.

object                   YYYYMMDD(UT)   mag      code
PNV_J180645.2-360458     20120803.49784 9.87Ic   Mhh.VSOLJ
PNV_J180645.2-360458     20120803.49878 10.54Rc  Mhh.VSOLJ
PNV_J180645.2-360458     20120803.50007 11.22V   Mhh.VSOLJ
PNV_J180645.2-360458     20120803.50229 12.33B   Mhh.VSOLJ
PNV_J180645.2-360458     20120803.50539 11.23y   Mhh.VSOLJ

----- Original Message -----
From: "Brian Skiff" <bas@lowell.edu>
To: "Hiroyuki Maehara" <maehara@kwasan.kyoto-u.ac.jp>
Cc: <vsnet-alert@ooruri.kusastro.kyoto-u.ac.jp>
Sent: Friday, August 03, 2012 11:43 PM
Subject: [vsnet-alert 14825] Re: PNV_J180645.2-360458

     I note also that although the object is faint in 2MASS images,
it is prominent in WISE images at its three shorter mid-IR wavelengths,
but fades again at 22 microns.  The WISE coordinates match that of the
current outburst accurately, and thus it is not the star ~6" northeast
that is prominent in 2MASS images.  The WISE detection implies some
sort of warm dust is involved, consistent with the earlier brightening.
     If you can examine WISE images, look at a 5'x5' field, and notice
the planetary nebula about 2'.5 southeast (perhaps mag 18 in the
visible), whose cool dust becomes very bright at the longer
wavelengths.  Interesting comparison!

\Brian

From: "Sebastián Otero" <varsao@hotmail.com>

To: <vsnet-alert@ooruri.kusastro.kyoto-u.ac.jp>; <vsnet-newvar@ooruri.kusastro.kyoto-u.ac.jp>

Sent: Saturday, August 04, 2012 5:39 PM

Subject: [vsnet-newvar 3628] Re: [vsnet-alert 14824] PNV_J180645.2-360458

> The object
> showed short fadings (time-scale of a few tens of days) in the 2009
> observing season.
> http://www.astrouw.edu.pl/cgi-asas/asas_variable/180645-3604.9,asas3,0,…

Impressive. Weird that it didn't make it to the ASAS catalogue. The onset of large amplitude variability at the most recent ASAS epochs is also intriguing. Maybe UXor-like and with an initial FUor-like initial eruption?

> Note: there is a mag~13-14 star at 18:06:45.67, -36:04:55.1 (J2000.0),
> about 6 arcseconds NE of this object.

The V-mag. of this object is around 14.4 and it is rather red (J-K= 1.14).

 > The long-term variation of the object is similar to those of V4334 Sgr
> and FG Sge (or very slow nova?). Could someone perform spectroscopy of
> the object? This object was at Vmag=11.2 (B-V=1.1) on Aug. 3.50.

The UCAC3 magnitude indicates a mag. around 17 in V so this object has indeed a very large amplitude.

> object                   YYYYMMDD(UT)   mag      code
> PNV_J180645.2-360458     20120803.49784 9.87Ic   Mhh.VSOLJ
> PNV_J180645.2-360458     20120803.49878 10.54Rc  Mhh.VSOLJ
> PNV_J180645.2-360458     20120803.50007 11.22V   Mhh.VSOLJ
> PNV_J180645.2-360458     20120803.50229 12.33B   Mhh.VSOLJ
> PNV_J180645.2-360458     20120803.50539 11.23y   Mhh.VSOLJ

Do these magnitudes include the 14.4 mag. companion?
At this bright maximum the influence in V wouldn't be apparent (only 0.06 mag.) but the colors could be screwed up.

Cheers,
Sebastian

-------------------------
Sebastian Otero
VSX Team
American Association of Variable Star Observers