Blaise Joseph Canzian

The optical afterglow and host galaxy of GRB 000926

We present the discovery of the Optical Transient (OT) of the long{duration gamma-ray burst GRB 000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R =1 9:36. The transient faded with ad ecay slope of about 1:7 during the rst two days after which the slope increased abruptly (within a few hours) to about 2:4. The light-curve started to flatten o after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R =2 3:87 0:15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coin- cident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Mller et al., in prep.).

Detection of an optical transient following the 13 March 2000 short/hard gamma-ray burst ?

We imaged the error box of a gamma-ray burst of the short (0.5 s), hard type (GRB 000313), with the BOOTES-1 experiment in southern Spain, starting 4 min after the-ray event, in the I-band. A bright optical transient (OT 000313) with I= 9:4 0:1 was found in the BOOTES-1 image, close to the error box (3) provided by BATSE. Late time VRIK 0 -band deep observations failed to reveal an underlying host galaxy. If the OT 000313 is related to the short, hard GRB 000313, this would be the first optical counterpart ever found for this kind of events (all counterparts to date have been found for bursts of the long, soft type). The fact that only prompt optical emission has been detected (but no afterglow emission at all, as supported by theoretical models) might explain why no optical counterparts have ever been found for short, hard GRBs. This fact suggests that most short bursts might occur in a low-density medium and favours the models that relate them to binary mergers in very low-density environments.

Distances of Dwarf Carbon Stars

Parallaxes are presented for a sample of 20 nearby dwarf carbon stars. The inferred luminosities cover almost two orders of magnitude. Their absolute magnitudes and tangential velocities confirm prior expectations that some originate in the Galactic disk, although more than half of this sample are halo stars. Three stars are found to be astrometric binaries, and orbital elements are determined; their semimajor axes are 1 -- 3 AU, consistent with the size of an AGB mass-transfer donor star.

CCD Parallaxes for 309 Late-type Dwarfs and Subdwarfs

New, updated, and/or revised CCD parallaxes determined with the Strand Astrometric Reflector at the Naval Observatory Flagstaff Station (NOFS) are presented. Included are results for 309 late-type dwarf and subdwarf stars observed over the 30+ years that the program operated. For 124 of the stars, parallax determinations from other investigators have already appeared in the literature and we compare the different results. Also included here is new or updated

L′ and M′ Photometry of Ultracool Dwarfs

VI

GRB 030329, light curve flattens.

photometry on the Johnson-Kron-Cousins system for all but a few of the faintest targets. Together with 2MASS

Distant White Dwarfs in the US Naval Observatory Flagstaff Station Parallax Sample.

JHK_s

Astrometry of Comets at USNO Flagstaff Station

near-infrared photometry, a sample of absolute magnitude versus color and color versus color diagrams are constructed. Since large proper motion was a prime criterion for targeting the stars, the majority turn out to be either M-type subdwarfs or late M-type dwarfs. The sample also includes 50 dwarf or subdwarf L-type stars, and four T dwarfs. Possible halo subdwarfs are identified in the sample based on tangential velocity, subluminosity, and spectral type. Residuals from the solutions for parallax and proper motion for several stars show evidence of astrometric perturbations.