62 research outputs found
Proper Motions of Dwarf Spheroidal Galaxies from Hubble Space Telescope Imaging. I: Method and a Preliminary Measurement for Fornax
This article presents and discusses a method for measuring the proper motions
of the Galactic dwarf spheroidal galaxies using images taken with the Hubble
Space Telescope. The method involves fitting an effective point spread function
to the image of a star or quasi-stellar object to determine its centroid with
an accuracy of about 0.005 pixel (0.25 milliarcseconds) -- an accuracy
sufficient to measure the proper motion of a dwarf spheroidal galaxy using
images separated by just a few years. The data consist of images, dithered to
reduce the effects of undersampling, taken at multiple epochs with the Space
Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science
fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor
dwarf spheroidal galaxies and each has at least one quasi-stellar object whose
identity has been established by other studies. The rate of change with time of
the centroids of the stars of the dwarf spheroidal with respect to the centroid
of the quasi-stellar object is the proper motion.
Four independent preliminary measurements of the proper motion of Fornax for
three fields agree within their uncertainties. The weighted average of these
measurements is mu_alpha = 49 +- 13 milliarcseconds/century and mu_delta = -59
+- 13 milliarcseconds/century. The Galactocentric velocity derived from the
proper motion implies that Fornax is near perigalacticon, may not be bound to
the Milky Way, and is not a member of any of the proposed streams of galaxies
and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way
must have a mass of at least (1.6 +- 0.8) x 10^{12} solar masses.Comment: 4 tables, 16 figures (22 postscript files), 30+ pages. Accepted for
publication in AJ. v2: Revised to remove a typo in the abstract and improve
the formattin
WISE J163940.83-684738.6: A Y Dwarf identified by Methane Imaging
We have used methane imaging techniques to identify the near-infrared
counterpart of the bright WISE source WISEJ163940.83-684738.6. The large proper
motion of this source (around 3.0arcsec/yr) has moved it, since its original
WISE identification, very close to a much brighter background star -- it
currently lies within 1.5" of the J=14.90+-0.04 star 2MASS16394085-6847446.
Observations in good seeing conditions using methane sensitive filters in the
near-infrared J-band with the FourStar instrument on the Magellan 6.5m Baade
telescope, however, have enabled us to detect a near-infrared counterpart. We
have defined a photometric system for use with the FourStar J2 and J3 filters,
and this photometry indicates strong methane absorption, which unequivocally
identifies it as the source of the WISE flux. Using these imaging observations
we were then able to steer this object down the slit of the FIRE spectrograph
on a night of 0.6" seeing, and so obtain near-infrared spectroscopy confirming
a Y0-Y0.5 spectral type. This is in line with the object's
near-infrared-to-WISE J3--W2 colour. Preliminary astrometry using both WISE and
FourStar data indicates a distance of 5.0+-0.5pc and a substantial tangential
velocity of 73+-8km/s. WISEJ163940.83-684738.6 is the brightest confirmed Y
dwarf in the WISE W2 passband and its distance measurement places it amongst
the lowest luminosity sources detected to date.Comment: Accepted for publication in The Astrophysical Journal, 20 September
201
Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574
While following up L dwarf candidates selected photometrically from the Two
Micron All Sky Survey, we uncovered an unusual object designated 2MASS
J01415823-4633574. Its optical spectrum exhibits very strong bands of vanadium
oxide but abnormally weak absorptions by titanium oxide, potassium, and sodium.
Morphologically such spectroscopic characteristics fall intermediate between
old, field early-L dwarfs (log(g)~5) and very late M giants (log(g)~0), leading
us to favor low gravity as the explanation for the unique spectral signatures
of this L dwarf. Such a low gravity can be explained only if this L dwarf is
much lower in mass than a typical old field L dwarf of similar temperature and
is still contracting to its final radius. These conditions imply a very young
age. Further evidence of youth is found in the near-infrared spectrum,
including a triangular-shaped H-band continuum reminiscent of young brown dwarf
candidates discovered in the Orion Nebula Cluster. Using the above information
along with comparisons to brown dwarf atmospheric and interior models, our
current best estimate is that this L dwarf has an age of 1-50 Myr and a mass of
6-25 M_Jupiter. The location of 2MASS 0141-4633 on the sky coupled with a
distance estimate of ~35 pc and the above age estimate suggests that this
object may be a brown dwarf member of either the 30-Myr-old Tucana/Horologium
Association or the ~12-Myr-old beta Pic Moving Group.Comment: Accepted for publication in the 10 March 2006 issue (volume 639) of
the Astrophysical Journa
Searching for Binary Y Dwarfs with the Gemini Multi-Conjugate Adaptive Optics System (GeMS)
The NASA Wide-field Infrared Survey Explorer (WISE) has discovered almost all the known members of the new class of Y-type brown dwarfs. Most of these Y dwarfs have been identified as isolated objects in the field. It is known that binaries with L- and T-type brown dwarf primaries are less prevalent than either M-dwarf or solar-type primaries, they tend to have smaller separations and are more frequently detected in near-equal mass configurations. The binary statistics for Y-type brown dwarfs, however, are sparse, and so it is unclear if the same trends that hold for L- and T-type brown dwarfs also hold for Y-type ones. In addition, the detection of binary companions to very cool Y dwarfs may well be the best means available for discovering even colder objects. We present results for binary properties of a sample of five WISE Y dwarfs with the Gemini Multi-Conjugate Adaptive Optics System. We find no evidence for binary companions in these data, which suggests these systems are not equal-luminosity (or equal-mass) binaries with separations larger than ~0.5–1.9 AU. For equal-mass binaries at an age of 5 Gyr, we find that the binary binding energies ruled out by our observations (i.e., 10^(42) erg) are consistent with those observed in previous studies of hotter ultra-cool dwarfs
Oscillation frequencies and mode lifetimes in alpha Centauri A
We analyse our recently-published velocity measurements of alpha Cen A
(Butler et al. 2004). After adjusting the weights on a night-by-night basis in
order to optimize the window function to minimize sidelobes, we extract 42
oscillation frequencies with l=0 to 3 and measure the large and small frequency
separations. We give fitted relations to these frequencies that can be compared
with theoretical models and conclude that the observed scatter about these fits
is due to the finite lifetimes of the oscillation modes. We estimate the mode
lifetimes to be 1-2 d, substantially shorter than in the Sun.Comment: Accepted by Ap
GNOSIS: the first instrument to use fibre Bragg gratings for OH suppression
GNOSIS is a prototype astrophotonic instrument that utilizes OH suppression
fibres consisting of fibre Bragg gratings and photonic lanterns to suppress the
103 brightest atmospheric emission doublets between 1.47-1.7 microns. GNOSIS
was commissioned at the 3.9-meter Anglo-Australian Telescope with the IRIS2
spectrograph to demonstrate the potential of OH suppression fibres, but may be
potentially used with any telescope and spectrograph combination. Unlike
previous atmospheric suppression techniques GNOSIS suppresses the lines before
dispersion and in a manner that depends purely on wavelength. We present the
instrument design and report the results of laboratory and on-sky tests from
commissioning. While these tests demonstrated high throughput and excellent
suppression of the skylines by the OH suppression fibres, surprisingly GNOSIS
produced no significant reduction in the interline background and the
sensitivity of GNOSIS and IRIS2 is about the same as IRIS2. It is unclear
whether the lack of reduction in the interline background is due to physical
sources or systematic errors as the observations are detector noise-dominated.
OH suppression fibres could potentially impact ground-based astronomy at the
level of adaptive optics or greater. However, until a clear reduction in the
interline background and the corresponding increasing in sensitivity is
demonstrated optimized OH suppression fibres paired with a fibre-fed
spectrograph will at least provide a real benefits at low resolving powers.Comment: 15 pages, 13 figures, accepted to A
New Y and T dwarfs from WISE identified by Methane Imaging
We identify new Y- and T-type brown dwarfs from the WISE All Sky data release
using images obtained in filters that divide the traditional near-infrared H
and J bands into two halves -- specifically CH4s & CH4l in the H and J2 & J3 in
the J. This proves to be very effective at identifying cool brown dwarfs via
the detection of their methane absorption, as well as providing preliminary
classification using methane colours and WISE-to-near-infrared colours. New and
updated calibrations between T/Y spectral types and CH4s-CH4l, J3-W2, and
CH4s-W2 colours are derived, producing classification estimates good to a few
spectral sub-types. We present photometry for a large sample of T and Y dwarfs
in these filters, together with spectroscopy for 23 new ultra-cool dwarfs - two
Y dwarfs and twenty one T dwarfs. We identify a further 8 new cool brown
dwarfs, which we have high confidence are T dwarfs based on their methane
photometry. We find that, for objects observed on a 4m-class telescope at J
band magnitudes of ~20 or brighter, CH4s-CH4l is the more powerful colour for
detecting objects and then estimating spectral types. Due to the lower sky
background in the J-band, the J3 and J2 bands are more useful for identifying
fainter cool dwarfs at J>22. The J3-J2 colour is poor at estimating spectral
types. But fortunately, once J3-J2 confirms that an object is a cool dwarf, the
J3-W2 colour is very effective at estimating approximate spectral types.Comment: 29 pages, 6 figures. To appear in the Astrophysical Journal
Supplemen
Detection of Planetary and Stellar Companions to Neighboring Stars via a Combination of Radial Velocity and Direct Imaging Techniques
13 pages, 6 figures, 4 tables, accepted for publication in the Astronomical Journal (submitted 25 Feb 2019; accepted 28 April 2019). Machine readable tables and Posteriors from the RadVel fits are available here: http://stephenkane.net/rvfits.tarThe sensitivities of radial velocity (RV) surveys for exoplanet detection are extending to increasingly longer orbital periods, where companions with periods of several years are now being regularly discovered. Companions with orbital periods that exceed the duration of the survey manifest in the data as an incomplete orbit or linear trend, a feature that can either present as the sole detectable companion to the host star, or as an additional signal overlain on the signatures of previously discovered companion(s). A diagnostic that can confirm or constrain scenarios in which the trend is caused by an unseen stellar rather than planetary companion is the use of high-contrast imaging observations. Here, we present RV data from the Anglo-Australian Planet Search (AAPS) for 20 stars that show evidence of orbiting companions. Of these, six companions have resolved orbits, with three that lie in the planetary regime. Two of these (HD 92987b and HD 221420b) are new discoveries. Follow-up observations using the Differential Speckle Survey Instrument (DSSI) on the Gemini South telescope revealed that 5 of the 20 monitored companions are likely stellar in nature. We use the sensitivity of the AAPS and DSSI data to place constraints on the mass of the companions for the remaining systems. Our analysis shows that a planetary-mass companion provides the most likely self-consistent explanation of the data for many of the remaining systems.Peer reviewedFinal Accepted Versio
A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages
Using a large sample of optical spectra of late-type dwarfs, we identify a
subset of late-M through L field dwarfs that, because of the presence of
low-gravity features in their spectra, are believed to be unusually young. From
a combined sample of 303 field L dwarfs, we find observationally that
7.6+/-1.6% are younger than 100 Myr. This percentage is in agreement with
theoretical predictions once observing biases are taken into account. We find
that these young L dwarfs tend to fall in the southern hemisphere (Dec < 0 deg)
and may be previously unrecognized, low-mass members of nearby, young
associations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus.
We use a homogeneously observed sample of roughly one hundred and fifty
6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-Resolution
Imaging Spectrometer at the W. M. Keck Observatory to examine the strength of
the 6708-A Li I line as a function of spectral type and further corroborate the
trends noted by Kirkpatrick et al. (2000). We use our low-gravity spectra to
investigate the strength of the Li I line as a function of age. The data weakly
suggest that for early- to mid-L dwarfs the line strength reaches a maximum for
a few 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L
dwarfs the line is weaker or undetectable. We show that a weakening of lithium
at lower gravities is predicted by model atmosphere calculations, an effect
partially corroborated by existing observational data. Larger samples
containing L dwarfs of well determined ages are needed to further test this
empirically. If verified, this result would reinforce the caveat first cited in
Kirkpatrick et al. (2006) that the lithium test should be used with caution
when attempting to confirm the substellar nature of the youngest brown dwarfs.Comment: 73 pages with 22 figures; to appear in ApJ (Dec 20, 2008, v689n2
issue
- …