9,040 research outputs found
1-1.4 Micron Spectral Atlas of Stars
We present a catalog of J-band (1.08 um to 1.35 um) stellar spectra at low
resolution (R ~ 400). The targets consist of 105 stars ranging in spectral type
from O9.5 to M7 and luminosity classes I through V. The relatively featureless
spectra of hot stars, earlier than A4, can be used to remove the atmospheric
features which dominate ground-based J-band spectroscopy. We measure equivalent
widths for three absorption lines and nine blended features which we identify
in the spectra. Using detailed comparison with higher resolution spectra, we
demonstrate that low resolution data can be used for stellar classification,
since several features depend on the effective temperature and gravity. For
example The CN index (1.096 - 1.104 um) decreases with temperature, but the
strength of a blended feature at 1.28 um (consisting of primarily P beta)
increases. The slope of a star's spectrum can also be used to estimate its
effective temperature. The luminosity class of a star correlates with the ratio
of the Mg I (1.1831 um) line to a blend of several species at 1.16 um. Using
these indicators, a star can be classified to within several subclasses.
Fifteen stars with particularly high and low metal abundances are included in
the catalog and some spectral dependence on metal abundance is also found.Comment: 35 pages, 10 figures (3a-e are in gif format. For complete high
resolution figures, go to http://www.astro.ucla.edu/~malkan/newjspec/) ;
Accepted for published in ApJS; For associated spectra files, see
http://www.astro.ucla.edu/~malkan/newjspec
K-Band Spectroscopy of an Obscured Massive Stellar Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)
with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the
vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using
NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the
near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the
highest surface brightness peak in the mid-IR (12-18 um) ISO image presented by
Mirabel et al (1998). We obtained high signal-to-noise 2.03-2.45 um spectra of
the nucleus and the obscured star cluster at R = 1900.
The cluster is very young (age ~ 4 Myr), massive (M ~ 16E6 M_sun), and
compact (density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius),
assuming a Salpeter IMF (0.1-100 M_sun). Its hot stars have a radiation field
characterized by T_eff ~ 39,000 K, and they ionize a compact HII region with
n_e ~ 10^4 cm^(-3). The stars are deeply embedded in gas and dust (A_V = 9-10
mag), and their strong FUV field powers a clumpy photodissociation region with
densities n_H > 10^5 cm^(-3) on scales of ~ 200 pc, radiating L{H_2 1-0 S(1)}=
9600 L_sun.Comment: 4 pages, 4 embedded figures, uses emulateapj.sty. To appear in ApJL.
Also available at http://astro.berkeley.edu/~agilber
Spectropolarimetry of the H-alpha line in Herbig Ae/Be stars
Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS
telescope, we have obtained a large number of high precision spectropolarimetrc
observations (284) of Herbig AeBe stars collected over 53 nights totaling more
than 300 hours of observing. Our sample of five HAeBe stars: AB Aurigae,
MWC480, MWC120, MWC158 and HD58647, all show systematic variations in the
linear polarization amplitude and direction as a function of time and
wavelength near the H-alpha line. In all our stars, the H-alpha line profiles
show evidence of an intervening disk or outflowing wind, evidenced by strong
emission with an absorptive component. The linear polarization varies by 0.2%
to 1.5% with the change typically centered in the absorptive part of the line
profile. These observations are inconsistent with a simple disk-scattering
model or a depolarization model which produce polarization changes centered on
the emmissive core. We speculate that polarized absorption via optical pumping
of the intervening gas may be the cause.Comment: Accepted for publication in ApJ Letter
Spectropolarimetric observations of Herbig Ae/Be Stars I: HiVIS spectropolarimetric calibration and reduction techniques
Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS
telescope in Hawaii, we are collecting a large number of high precision
spectropolarimetrc observations of stars. In order to precisely measure very
small polarization changes, we have performed a number of polarization
calibration techniques on the AEOS telescope and HiVIS spectrograph. We have
extended our dedicated IDL reduction package and have performed some hardware
upgrades to the instrument. We have also used the ESPaDOnS spectropolarimeter
on CFHT to verify the HiVIS results with back-to-back observations of MWC 361
and HD163296. Comparision of this and other HiVIS data with stellar
observations from the ISIS and WW spectropolarimeters in the literature further
shows the usefulness of this instrument.Comment: 35 pages, 44 figures, Accepted by PAS
Hot Stars and Cool Clouds: The Photodissociation Region M16
We present high-resolution spectroscopy and images of a photodissociation
region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II
telescope. PDRs play a significant role in regulating star formation, and M16
offers the opportunity to examine the physical processes of a PDR in detail. We
simultaneously observe both the molecular and ionized phases of the PDR and
resolve the spatial and kinematic differences between them. The most prominent
regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars
is the primary excitation source, although collisions also preferentially
populate the lowest vibrational levels of H2. Variations in density-sensitive
emission line ratios demonstrate that the molecular cloud is clumpy, with an
average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the
molecular region directly and find T_H2 = 930 K. The observed density,
temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the
ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions
of the PDR, the recombination line widths include a non-thermal component,
which we attribute to viewing geometry.Comment: 5 pages including 2 Postscript figures. To appear in ApJ Letters,
April 200
The Gibbs-Thomson formula at small island sizes - corrections for high vapour densities
In this paper we report simulation studies of equilibrium features, namely
circular islands on model surfaces, using Monte-Carlo methods. In particular,
we are interested in studying the relationship between the density of vapour
around a curved island and its curvature-the Gibbs-Thomson formula. Numerical
simulations of a lattice gas model, performed for various sizes of islands,
don't fit very well to the Gibbs-Thomson formula. We show how corrections to
this form arise at high vapour densities, wherein a knowledge of the exact
equation of state (as opposed to the ideal gas approximation) is necessary to
predict this relationship. Exploiting a mapping of the lattice gas to the Ising
model one can compute the corrections to the Gibbs-Thomson formula using high
field series expansions. We also investigate finite size effects on the
stability of the islands both theoretically and through simulations. Finally
the simulations are used to study the microscopic origins of the Gibbs-Thomson
formula. A heuristic argument is suggested in which it is partially attributed
to geometric constraints on the island edge.Comment: 27 pages including 7 figures, tarred, gzipped and uuencoded. Prepared
using revtex and espf.sty. To appear in Phys. Rev.
Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
We present initial results of a deep near-IR spectroscopic survey covering
the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the
Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z <
2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 <
log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band
atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in
the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight,
relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared
to local galaxies. Using photoionization models, we argue that the offset of
the z~2.3 locus relative to z~ 0 is explained by a combination of harder
ionizing radiation field, higher ionization parameter, and higher N/O at a
given O/H than applies to most local galaxies, and that the position of a
galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to
gas-phase oxygen abundance. The observed nebular emission line ratios are most
easily reproduced by models in which the net ionizing radiation field resembles
a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to
the solar value at all O/H. We critically assess the applicability of
commonly-used strong line indices for estimating gas-phase metallicities, and
consider the implications of the small intrinsic scatter in the empirical
relationship between excitation-sensitive line indices and stellar mass (i.e.,
the "mass-metallicity" relation), at z~2.3.Comment: 41 pages, 25 figures, accepted for publication in the Astrophysical
Journal. Version with full-resolution figures available at
http://www.astro.caltech.edu/~ccs/mos_bpt_submit.pd
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