1,642 research outputs found
Precise Ages of Field Stars from White Dwarf Companions
Observational tests of stellar and Galactic chemical evolution call for the
joint knowledge of a star's physical parameters, detailed element abundances,
and precise age. For cool main-sequence (MS) stars the abundances of many
elements can be measured from spectroscopy, but ages are very hard to
determine. The situation is different if the MS star has a white dwarf (WD)
companion and a known distance, as the age of such a binary system can then be
determined precisely from the photometric properties of the cooling WD. As a
pilot study for obtaining precise age determinations of field MS stars, we
identify nearly one hundred candidates for such wide binary systems: a faint WD
whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a
good parallax (). We model the WD's multi-band
photometry with the BASE-9 code using this precise distance (assumed to be
common for the pair) and infer ages for each binary system. The resulting age
estimates are precise to () for () MS-WD systems.
Our analysis more than doubles the number of MS-WD systems with precise
distances known to date, and it boosts the number of such systems with precise
age determination by an order of magnitude. With the advent of the Gaia DR2
data, this approach will be applicable to a far larger sample, providing ages
for many MS stars (that can yield detailed abundances for over 20 elements),
especially in the age range 2 to 8\,\Gyr, where there are only few known star
clusters.Comment: 9 pages, 5 figures, 1 catalog; Submitted to Ap
Galaxy-Galaxy Lensing in the Hubble Deep Field: The Halo Tully-Fisher Relation at Intermediate Redshift
A tangential distortion of background source galaxies around foreground lens
galaxies in the Hubble Deep Field is detected at the 99.3% confidence level. An
important element of our analysis is the use of photometric redshifts to
determine distances of lens and source galaxies and rest-frame B-band
luminosities of the lens galaxies. The lens galaxy halos obey a Tully-Fisher
relation between halo circular velocity and luminosity; the typical lens
galaxy, at a redshift z = 0.6, has a circular velocity of 210 +/-40 km/s at M_B
= -18.5, if q_0 = 0.5. Control tests, in which lens and source positions and
source ellipticities are randomized, confirm the significance level of the
detection quoted above. Furthermore, a marginal signal is also detected from an
independent, fainter sample of source galaxies without photometric redshifts.
Potential systematic effects, such as contamination by aligned satellite
galaxies, the distortion of source shapes by the light of the foreground
galaxies, PSF anisotropies, and contributions from mass distributed on the
scale of galaxy groups are shown to be negligible. A comparison of our result
with the local Tully-Fisher relation indicates that intermediate-redshift
galaxies are fainter than local spirals by 1.0 +/- 0.6 B mag at a fixed
circular velocity. This is consistent with some spectroscopic studies of the
rotation curves of intermediate-redshift galaxies. This result suggests that
the strong increase in the global luminosity density with redshift is dominated
by evolution in the galaxy number density.Comment: Revised version with minor changes. 13 pages, 7 figures, LaTeX2e,
uses emulateapj and multicol styles (included). Accepted by Ap
Results from the CASTLES Survey of Gravitational Lenses
We show that most gravitational lenses lie on the passively evolving
fundamental plane for early-type galaxies. For burst star formation models (1
Gyr of star formation, then quiescence) in low Omega_0 cosmologies, the stellar
populations of the lens galaxies must have formed at z_f > 2. Typical lens
galaxies contain modest amounts of patchy extinction, with a median
differential extinction for the optical (radio) selected lenses of E(B-V) =
0.04 (0.07) mag. The dust can be used to determine both extinction laws and
lens redshifts. For example, the z_l=0.96 elliptical lens in MG0414+0534 has an
R_V=1.7 +/- 0.1 mean extinction law. Arc and ring images of the quasar and AGN
source host galaxies are commonly seen in NICMOS H band observations. The hosts
are typically blue, L < L_* galaxies.Comment: 12 pages, 10 figures, from Proceedings of the 9th Annual Astrophysics
Conference in Maryland, After the Dark Ages: When Galaxies Were Youn
A Young Stellar Cluster in the Nucleus of NGC 4449
We have obtained 1-2 A resolution optical Echellette spectra of the nuclear
star cluster in the nearby starburst galaxy NGC 4449. The light is clearly
dominated by a very young (6-10 Myr) population of stars. For our age dating,
we have used recent population synthesis models to interpret the observed
equivalent width of stellar absorption features such as the HI Balmer series
and the CaII triplet around 8500 A. We also compare the observed spectrum of
the nuclear cluster to synthesized spectra of simple stellar populations of
varying ages. All these approaches yield a consistent cluster age. Metallicity
estimates based on the relative intensities of various ionization lines yield
no evidence for significant enrichment in the center of this low mass galaxy:
the metallicity of the nuclear cluster is about one fourth of the solar value,
in agreement with independent estimates for the disk material of NGC 4449.Comment: 24 pages (incl. 7 figures), accepted by AJ, March 2001 issue revised
version with minor changes and additions, one additional figur
Knowing what you need to know about needs assessment
This paper reviews the scientific literature on needs assessment of individuals living in the community. Providing an overview of the field with a focus on capturing useful information for planning health interventions in the community, this paper will highlight: The current policy context that underpins the importance of needs assessment in health care; Key papers and basic theoretical concepts that can guide a coherent approach to community care needs assessment, including Bradshaw, 1972 and Stevens & Gabbay, 1991; The development of semi-structured interviews in mental health as a focus for designing new tools - with the Camberwell Assessment of Need (CAN) being the most well known; How to deal with the organisational issues involved with the assessment of unmet need in the community; The current evidence base for developing a national approach to needs assessment; And a brief look at the latest self-reported needs assessment instruments
More evidence for hidden spiral and bar features in bright early-type dwarf galaxies
Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000),
we present further evidence that a sizable fraction of bright early-type dwarfs
in the Virgo cluster are genuine disk galaxies, or are hosting a disk
component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed
with the Very Large Telescope in and , we found another four systems
exhibiting non-axisymmetric structures, such as a bar and/or spiral arms,
indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable
are the two-armed spiral pattern in IC0783 and the bar and trailing arms in
NGC4431. For both galaxies the disk nature has recently been confirmed by a
rotation velocity measurement (Simien & Prugniel 2002). Our photometric search
is based on a Fourier decomposition method and a specific version of unsharp
masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former
late-type galaxies which were transformed to early-type morphology, e.g. by
``harassment'', during their infall to the cluster, while maintaining part of
their disk structure.Comment: A&A accepte
Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey
We report preliminary results of a deep near-infrared search for
methane-absorbing brown dwarfs; almost five years after the discovery of Gl
229b, there are only a few confirmed examples of this type of object. New J
band, wide-field images, combined with pre-existing R band observations, allow
efficient identification of candidates by their extreme (R-J) colours.
Follow-up measurements with custom filters can then confirm objects with
methane absorption. To date, we have surveyed a total of 11.4 square degrees to
J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in
1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90%
confidence, this implies that the space density of objects similar to Gl 229b
is less than 0.012 per cubic parsec. These calculations account for the
vertical structure of the Galaxy, which can be important for sensitive
measurements. Combining published theoretical atmospheric models with our
observations sets an upper limit of alpha <= 0.8 for the exponent of the
initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter
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