Close Double-Stars Discovered among Photometric Standards with Gemini Altair Adaptive Optics System: FS108 and GSPC P309-U.

ATel #1729; Silas Laycock, Anil Dosaj & Michael Hoenig (Gemini Observatory)
on 22 Sep 2008; 20:43 UT
Credential Certification: Silas Laycock (silas@gemini.edu)

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Reported on behalf of Gemini Observatory.

In the course of routine photometric calibrations carried out at Gemini
North (Mauna Kea, Hawaii), we have discovered that two standard stars (FS 108 and GSPC P309-U)
are in fact close double-stars. We report our findings in order to alert the community
that these stars are not suitable standards .The observations were made with the facility
Adaptive Optics system (Altair) and near infrared imager (NIRI). Altair was operating in
Natural guide-star (NGS) mode, with the field-lens in the beam. For standard star observations
the target is on-axis and is used to sense the atmospheric aberrations.

FS 108 located at co-ordinates RA=03:01:09.8, Dec= +46:58:47.85 was observed
on 19-Aug-2008 at 15:21UT. A sequence of 9 images was taken in the J-filter.
AO corrections were performed at 1 kHz. The natural uncorrected seeing was 0.48"
(in the visible) at airmass = 1.124. The f/14 beam provided an image scale of 0.05 arcsec/pixel.
The system delivered a PSF with FWHM=0.178" (at 1.6 microns)

FS 108 was found to be a close double-star, with components (FS108-A &
FS108-B) separated by 0.34 arcsec and differing in brightness by 0.62 mag.
The unresolved blend has 2MASS magnitudes J=10.086(0.020), H=9.786(0.017),
K=9.710 (0.016). We used the 2MASS J-band magnitude to calibrate the combined
flux measured from the two stars in our NIRI images. The resulting zero-points
were then used to compute J-magnitudes for the individual components in each image.
We report the following results: (Mean of measurements from the 9 images, followed
by the total spread.) We note that FS 108 was observed with Altair again on 15-Sep-2008
(in slightly poorer conditions) confirming these results.

FS-108-A: J=11.345 (spread=0.033)
FS-108-B: J=11.969 (spread=0.061)
Separation: 0.3385 arcsec (spread=0.0099")
Position Angle: 0 degrees (Star-B is Due North of Star-A)


GSPC P309-U located at co-ordinates RA=07:30:34.6, Dec= +29:51:12 (J2000)
was observed on 15-Sep-2008 at 15:21 UT. AO corrections were performed at 200 Hz.
The natural uncorrected seeing was 0.65" (in the visible) at airmass = 1.3. The f/32 beam
provided an image scale of 0.022 arcsec/pixel. The system delivered a PSF with
FWHM=0.08" (at 2.2 microns)

GSPC P309-U was found to be a close double-star, with components (GSPC P309-U-A
& GSPC P309-U-B) separated by 0.33 arcsec and differing in brightness by 0.72 mag.
The nearest 2MASS object is 2MASS J07303490+2950044 at 07:30:34.9, +29:50:04
(J2000), which we propose is the same star, with the coords shifted by proper motion.
The 2MASS magnitudes are J=11.891(0.02), H=11.52(0.029), K=11.434(0.021). We used
the 2MASS K-band magnitude to calibrate the combined flux measured from the two
stars in our NIRI images. The resulting zero-points were then used to compute K-magnitudes
for the individual components in each image. From 4 images (a fifth image had a defect partially
covering star-B, and was rejected) we report the following results. (Mean of measurements
followed by the total spread)

P309-U-A: K=11.8846 (spread=0.0086)
P309-U-B: K=12.6063 (spread=0.0169)
Separation: 0.32 arcsec
Position Angle (East of North): 332.65 degrees (spread=0.85)