1,210 research outputs found
Transformations between the theoretical and observational planes in the HST-NICMOS and WFPC2 photometric systems
Color-temperature relations and bolometric corrections in the HST-NICMOS
F1110W, F160W and F222M and in the WFPC2 F439W, F555W and F814W photometric
systems, using two different sets of model atmospheres, have been derived. This
database of homogeneous, self-consistent transformations between the
theoretical and observational planes also allows combinations of visual and
infrared quantities, without any further transformation between the two
different photometric systems. The behavior of the inferred quantities with
varying the stellar parameters, the adopted model atmospheres and the
instrumental configurations are investigated. Suitable relations to transform
colors and bolometric corrections from HST to ground-based photometric systems
are also provided.Comment: 22 pages, 14 figure
The calculation of longitude and latitude from geodesic measurements
The solution of the geodesic problem for an oblate ellipsoid is developed in
terms of series. Tables are provided to simplify the computation. [This is an
English translation of F. W. Bessel, Astronomische Nachrichten 4(86), 241-254
(1825). The mathematical notation has been updated to conform to current
conventions and, in a few places, the equations have been rearranged for
clarity. Several errors have been corrected, a figure has been included, and
the tables have been recomputed.]Comment: 11 pages, including 1 figure and 4 pages of tables. Version 2 and 3
fix some minor errors. This translation was edited by Charles F. F. Karney
and Rodney E. Deakin. A transcription of the original paper is available at
arXiv:0908.1823 . For links to other 18th and 19th century papers on
geodesics, see
http://geographiclib.sourceforge.net/geodesic-papers/biblio.htm
SearchCal: a Virtual Observatory tool for searching calibrators in optical long baseline interferometry. I: The bright object case
In long baseline interferometry, the raw fringe contrast must be calibrated
to obtain the true visibility and then those observables that can be
interpreted in terms of astrophysical parameters. The selection of suitable
calibration stars is crucial for obtaining the ultimate precision of
interferometric instruments like the VLTI. We have developed software SearchCal
that builds an evolutive catalog of stars suitable as calibrators within any
given user-defined angular distance and magnitude around the scientific target.
We present the first version of SearchCal dedicated to the bright-object case
V<=10; K<=5). Star catalogs available at the CDS are consulted via web
requests. They provide all the useful information for selecting of calibrators.
Missing photometries are computed with an accuracy of 0.1 mag and the missing
angular diameters are calculated with a precision better than 10%. For each
star the squared visibility is computed by taking the wavelength and the
maximum baseline of the foreseen observation into account.} SearchCal is
integrated into ASPRO, the interferometric observing preparation software
developed by the JMMC, available at the address: http://mariotti.fr
Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)
The current classification system of M stars on the main sequence
distinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, and
extreme subdwarfs - esdM). The spectroscopic definition of these classes is
based on the relative strength of prominent CaH and TiO molecular absorption
bands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, and
TiO5). We re-examine this classification system in light of our ongoing
spectroscopic survey of stars with proper motion \mu > 0.45 "/yr, which has
increased the census of spectroscopically identified metal-poor M stars to over
400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest
deficiencies in the current classification system. Observations of common
proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries
in the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours,
leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We
propose a revision of the classification system based on an empirical
calibration of the TiO/CaH ratio for stars of near solar metallicity. We
introduce the parameter \zeta_{TiO/CaH} which quantifies the weakening of the
TiO bandstrength due to metallicity effect, with values ranging from
\zeta_{TiO/CaH}=1 for stars of near-solar metallicity to \zeta_{TiO/CaH}~0 for
the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity
classes based on the value of the parameter \zeta_{TiO/CaH}; and refine the
scheme by introducing an additional class of ultra subdwarfs (usdM). We
introduce sequences of sdM, esdM, and usdM stars to be used as formal
classification standards.Comment: 15 pages, accepted for publication in the Astrophysical Journa
Membership, lithium, and metallicity in the young open clusters IC 2602 and IC 2391: enlarging the sample
We present lithium abundances for ~50 X-ray selected candidate members of the
30-50 Myr old open clusters IC 2602 and IC 2391. These data enlarge and extend
to cooler temperatures previous Li surveys of these clusters by Stauffer et al.
(1989) and Randich et al. (1997). We also give for the first time an estimate
of the metallicity of the two clusters which turns out to be close to solar.
Radial velocity measurements together with Halpha chromospheric emission and
the presence/absence of other spectroscopic features are used to ascertain the
membership status for the sample stars not yet confirmed as cluster members;
rotational velocities have also been determined for all sample stars. Stars
more massive than ~1 Mo in both clusters show no sign of significant Li
depletion, while lower mass stars are all lithium depleted, with the amount of
Li depletion increasing to cooler temperatures. We confirm that the late G and
early K stars in IC 2602 present a star-to-star scatter in Li abundances
similar to, but not as large as the one in the Pleiades. A scatter is also seen
among late-K and M dwarfs. Unlike in the Pleiades and Alpha Per clusters, the
scatter among early-K stars in IC 2602 shows only marginal correlation with
rotation. Our data suggest that the drop-off of lithium towards lower masses
may start at an earlier color in IC 2391 than in IC 2602, but larger cluster
samples are needed to confirm this result. In addition, whereas G and early K
stars in the two clusters are, on average, more Li rich than their counterparts
in the Pleiades, a fraction of the coolest stars, in particular in IC 2391, are
as depleted as as the lowest-Li Pleiades stars of the same mass. If they
continue depleting Li on their way to the main sequence, they are expected to
be more Li depleted than the Pleiades at the age of the latter cluster.Comment: to appear in A&
- …