101 research outputs found
Dynamical Masses of Young Stars in Multiple Systems
We present recent measurements of the orbital motion in the young binaries DF
Tau and ZZ Tau, and the hierarchical triple Elias 12, that were obtained with
the Fine Guidance Sensors on the HST and at the Keck Observatory using adaptive
optics. Combining these observations with previous measurements from the
literature, we compute preliminary orbital parameters for DF Tau and ZZ Tau. We
find that the orbital elements cannot yet be determined precisely because the
orbital coverage spans only about 90 degr in position angle. Nonetheless, the
range of possible values for the period and semi-major axis already defines a
useful estimate for the total mass in DF Tau and ZZ Tau, with values of
0.90{+0.85}{-0.35} M_sun and 0.81{+0.44}{-0.25} M_sun, respectively, at a
fiducial distance of 140 pc.Comment: 26 pages, 9 figures, accepted for publication in A
Two Active Nuclei in 3C294
The z=1.786 radio galaxy 3C294 lies <10 arsec from a 12th mag star and has
been the target of at least three previous investigations using adaptive-optics
imaging. A major problem in interpreting these results is the uncertainty in
the precise alignment of the radio structure with the H or K-band AO imaging.
Here we report observations of the position of the AO guide star with the HST
Fine Guidance Sensor, which, together with positions from the U. S. Naval
Observatory's UCAC2 catalog, allow us to register the infrared and radio frames
to an accuracy of better than 0.1 arcsec. The result is that the nuclear
compact radio source is not coincident with the brightest discrete object in
the AO image, an essentially unresolved source on the eastern side of the light
distribution, as Quirrenbach et al. (2001) had suggested. Instead, the radio
source is centered about 0.9 arcsec to the west of this object, on one of the
two apparently real peaks in a region of diffuse emission. Nevertheless, the
conclusion of Quirrenbach et al. that 3C294 involves an ongoing merger appears
to be correct: analysis of a recent deep Chandra image of 3C294 obtained from
the archive shows that the nucleus comprises two X-ray sources, which are
coincident with the radio nucleus and the eastern stellar object. The
X-ray/optical flux ratio of the latter makes it extremely unlikely that it is a
foreground Galactic star.Comment: 16 pages, including 4 figures. To appear in ApJ, vol. 60
Pismis 24-1: The stellar upper mass limit preserved
Is there a stellar upper mass limit? Recent statistical work seems to indicate that there is and that it is in the vicinity of 150 M⊙. In this paper we use HST and ground-based data to investigate the brightest members of the cluster Pismis 24, one of which (Pismis 24-1) was previously inferred to have a mass greater than 200 M⊙, in apparent disagreement with that limit. We determine that Pismis 24-1 is composed of at least three objects, the resolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis 24-1NE. The evolutionary zero-age masses of Pismis 24-1SW, the unresolved system Pismis 24-1NE, and the nearby star Pismis 24-17 are all ≈100 Mȯ, very large but under the stellar upper mass limit.Facultad de Ciencias Astronómicas y GeofÃsica
JWST Fine Guidance Sensor Calibration
Abstract. The primary function of the Fine Guidance Sensor (FGS) is to provide continuous pointing information to the Observatory. The FGS will image two separate regions of the JWST field of view onto two independent 2k x 2k infrared focal planes arrays, which will cover a wavelength range of 0.6 to 5 µm. Once in fine guiding, the FGS will provide continuous pointing information to a precision of ≤ 5 milli-arc seconds at an update rate of 16 Hz for J 18.5 magnitude stars. In order to fulfill its functions, i.e., Identification, Acquisition, Track, and Fine Guide, the FGS will require calibration. In this paper, we present a brief overview of the FGS design, its operations, and its calibration
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