26 research outputs found
Dust Emission from Herbig Ae/Be stars - Evidence for Disks and Envelopes
IR and mm-wave emission from Herbig Ae/Be stars has produced conflicting
conclusions regarding the dust geometry in these objects. We show that the
compact dimensions of the mm-wave emitting regions are a decisive indication
for disks. But a disk cannot explain the spectral energy distribution (SED)
unless it is embedded in an extended envelope that (1) dominates the IR
emission and (2) provides additional disk heating on top of the direct stellar
radiation. Detailed radiative transfer calculations based on the simplest model
for envelope-embedded disks successfully fit the data from UV to mm wavelengths
and show that the disks have central holes. This model also resolves naturally
some puzzling results of IR imaging.Comment: 9 pages, 2 figures. accepted to ApJ
Infrared Classification of Galactic Objects
Unbiased analysis shows that IRAS data reliably differentiate between the
early and late stages of stellar evolution because objects at these stages
clearly segregate in infrared color-color diagrams. Structure in these diagrams
is primarily controlled by the density distribution of circumstellar dust. The
density profile around older objects is the steepest, declining as ,
while young objects have profiles that vary as and flatter. The
different density profiles reflect the different dynamics that govern the
different environments. Our analysis also shows that high mass star formation
is strongly concentrated within \about 5 kpc around the Galactic center, in
support of other studies.Comment: 11 pages, 3 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journal Letter
AGN Dusty Tori: II. Observational Implications of Clumpiness
From extensive radiative transfer calculations we find that clumpy torus
models with \No \about 5--15 dusty clouds along radial equatorial rays
successfully explain AGN infrared observations. The dust has standard Galactic
composition, with individual cloud optical depth \tV \about 30--100 at visual.
The models naturally explain the observed behavior of the 10\mic silicate
feature, in particular the lack of deep absorption features in AGN of any type.
The weak 10\mic emission feature tentatively detected in type 2 QSO can be
reproduced if in these sources \No drops to \about 2 or \tV exceeds \about 100.
The clouds angular distribution must have a soft-edge, e.g., Gaussian profile,
the radial distribution should decrease as or . Compact tori can
explain all observations, in agreement with the recent interferometric evidence
that the ratio of the torus outer to inner radius is perhaps as small as \about
5--10. Clumpy torus models can produce nearly isotropic IR emission together
with highly anisotropic obscuration, as required by observations. In contrast
with strict variants of unification schemes where the viewing-angle uniquely
determines the classification of an AGN into type 1 or 2, clumpiness implies
that it is only a probabilistic effect; a source can display type 1 properties
even from directions close to the equatorial plane. The fraction of obscured
sources depends not only on the torus angular thickness but also on the cloud
number \No. The observed decrease of this fraction at increasing luminosity can
be explained with a decrease of either torus angular thickness or cloud number,
but only the latter option explains also the possible emergence of a 10\mic
emission feature in QSO2.Comment: To appear in ApJ September 20, 200
AGN Dusty Tori: I. Handling of Clumpy Media
According to unified schemes of Active Galactic Nuclei (AGN), the central
engine is surrounded by dusty, optically thick clouds in a toroidal structure.
We have recently developed a formalism that for the first time takes proper
account of the clumpy nature of the AGN torus. We now provide a detailed report
of our findings in a two-paper series. Here we present our general formalism
for radiative transfer in clumpy media and construct its building blocks for
the AGN problem -- the source functions of individual dusty clouds heated by
the AGN radiation field. We show that a fundamental difference from smooth
density distributions is that in a clumpy medium, a large range of dust
temperatures coexist at the same distance from the radiation central source.
This distinct property explains the low dust temperatures found close to the
nucleus of NGC1068 in 10 \mic interferometric observations. We find that
irrespective of the overall geometry, a clumpy dust distribution shows only
moderate variation in its spectral energy distribution, and the 10\mic\
absorption feature is never deep. Furthermore, the X-ray attenuating column
density is widely scattered around the column density that characterizes the IR
emission. All of these properties are characteristic of AGN observations. The
assembly of clouds into AGN tori and comparison with observations is presented
in the companion paper.Comment: To appear in ApJ September 20, 200
Candidate Disk Wide Binaries in the Sloan Digital Sky Survey
Using SDSS Data Release 6, we construct two independent samples of candidate
stellar wide binaries selected as i) pairs of unresolved sources with angular
separation in the range 3'' - 16'', ii) common proper motion pairs with 5'' -
30'' angular separation, and make them publicly available. These samples are
dominated by disk stars, and we use them to constrain the shape of the
main-sequence photometric parallax relation M_r(r-i) and to study the
properties of wide binary systems. We estimate M_r(r-i) by searching for a
relation that minimizes the difference between distance moduli of primary and
secondary components of wide binary candidates. We model M_r(r-i) by a fourth
degree polynomial and determine the coefficients using Markov Chain Monte Carlo
fitting, independently for each sample. Aided by the derived photometric
parallax relation, we construct a series of high-quality catalogs of candidate
main-sequence binary stars. Using these catalogs, we study the distribution of
semi-major axes of wide binaries, a, in the 2,000 < a < 47,000 AU range. We
find the observations to be well described by the Opik distribution, f(a)~1/a,
for a<a_{break}, where a_{break} increases roughly linearly with the height Z
above the Galactic plane (a_{break}~12,300 Z[kpc]^0.7 AU). The number of wide
binary systems with 100 AU < a < a_{break}, as a fraction of the total number
of stars, decreases from 0.9% at Z=0.5 kpc to 0.5% at Z=3 kpc. The probability
for a star to be in a wide binary system is independent of its color. Given
this color, the companions of red components seem to be drawn randomly from the
stellar luminosity function, while blue components have a larger blue-to-red
companion ratio than expected from luminosity function.Comment: emulateApJ, 47 pages, 28 figures, accepted to Ap
Analysis of Stars Common to the IRAS and HIPPARCOS Surveys
For about 11,000 stars observed in the HIPPARCOS Survey and detected by IRAS
we calculate bolometric luminosities by integrating their spectral energy
distributions from the B band to far-IR wavelengths. We present an analysis of
the dependence of dust emission on spectral type and correlations between the
luminosity and dust emission for about 1000 sources with the best data
(parallax error less than 30%, error in luminosity of about 50% or better).
This subsample includes stars of all spectral types and is dominated by K and M
giants.
We use the IRAS [25]-[12] color to select stars with emission from
circumstellar dust and show that they are found throughout the
Hertzsprung-Russell diagram, including on the main sequence. Clear evidence is
found that M giants with dust emission have luminosities about 3 times larger
(about 3000 Lsun) than their counterparts without dust, and that mass loss on
the asymptotic giant branch for both M and C stars requires a minimum
luminosity of order 2000 Lsun. Above this threshold the mass-loss rate seems to
be independent of, or only weakly dependent on, luminosity. We also show that
the mass-loss rate for these stars is larger than the core mass growth rate,
indicating that their evolution is dominated by mass loss.Comment: Accepted to ApJ, 14 pages, 5 figure
Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. I. Crowded Field Photometry and Cluster Fiducial Sequences in ugriz
We present photometry for globular and open cluster stars observed with the
Sloan Digital Sky Survey (SDSS). In order to exploit over 100 million stellar
objects with r < 22.5 mag observed by SDSS, we need to understand the
characteristics of stars in the SDSS ugriz filters. While star clusters provide
important calibration samples for stellar colors, the regions close to globular
clusters, where the fraction of field stars is smallest, are too crowded for
the standard SDSS photometric pipeline to process. To complement the SDSS
imaging survey, we reduce the SDSS imaging data for crowded cluster fields
using the DAOPHOT/ALLFRAME suite of programs and present photometry for 17
globular clusters and 3 open clusters in a SDSS value-added catalog. Our
photometry and cluster fiducial sequences are on the native SDSS 2.5-meter
ugriz photometric system, and the fiducial sequences can be directly applied to
the SDSS photometry without relying upon any transformations. Model photometry
for red giant branch and main-sequence stars obtained by Girardi et al. cannot
be matched simultaneously to fiducial sequences; their colors differ by
~0.02-0.05 mag. Good agreement (< ~0.02 mag in colors) is found with Clem et
al. empirical fiducial sequences in u'g'r'i'z' when using the transformation
equations in Tucker et al.Comment: 30 pages, 25 figures. Accepted for publication in ApJS. Version with
high resolution figures available at
http://www.astronomy.ohio-state.edu/~deokkeun/AnJohnson.pd
Spectroscopic Properties of Cool Stars in the SDSS: An Analysis of Magnetic Activity and a Search for Subdwarfs
We present a spectroscopic analysis of nearly 8000 late-type dwarfs in the
Sloan Digital Sky Survey. Using the Halpha emission line as an activity
indicator, we investigate the fraction of active stars as a function of
spectral type and find a peak near type M8, confirming previous results. In
contrast to past findings, we find that not all M7-M8 stars are active. We show
that this may be a selection effect of the distance distributions of previous
samples, as the active stars appear to be concentrated near the Galactic Plane.
We also examine the activity strength (ratio of the luminosity emitted in
Halpha to the bolometric luminosity) for each star, and find that the mean
activity strength is constant over the range M0-M5 and declines at later types.
The decline begins at a slightly earlier spectral type than previously found.
We explore the effect that activity has on the broadband photometric colors and
find no significant differences between active and inactive stars. We also
carry out a search for subdwarfs using spectroscopic metallicity indicators,
and find 60 subdwarf candidates. Several of these candidates are near the
extreme subdwarf boundary. The spectroscopic subdwarf candidates are redder by
\~0.2 magnitudes in g-r compared to disk dwarfs at the same r-i color.Comment: 17 pages, 9 figures, accepted for publication in A
Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones
We perform an extensive test of theoretical stellar models for main-sequence
stars in ugriz, using cluster fiducial sequences obtained in the previous paper
of this series. We generate a set of isochrones using the Yale Rotating
Evolutionary Code (YREC) with updated input physics, and derive magnitudes and
colors in ugriz from MARCS model atmospheres. These models match cluster main
sequences over a wide range of metallicity within the errors of the adopted
cluster parameters. However, we find a large discrepancy of model colors at the
lower main sequence (Teff < ~4500 K) for clusters at and above solar
metallicity. We also reach similar conclusions using the theoretical isochrones
of Girardi et al. and Dotter et al., but our new models are generally in better
agreement with the data. Using our theoretical isochrones, we also derive
main-sequence fitting distances and turn-off ages for five key globular
clusters, and demonstrate the ability to derive these quantities from
photometric data in the Sloan Digital Sky Survey. In particular, we exploit
multiple color indices (g - r, g - i, and g - z) in the parameter estimation,
which allows us to evaluate internal systematic errors. Our distance estimates,
with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are
consistent with Hipparcos-based subdwarf fitting distances derived in the
Johnson-Cousins or Stromgren photometric systems.Comment: 26 pages, 28 figures. Accepted for publication in ApJ. Version with
high resolution figures available at
http://spider.ipac.caltech.edu/~deokkeun/sdss_iso.pd
Baryon Acoustic Oscillations in the Sloan Digital Sky Survey Data Release 7 Galaxy Sample
The spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) galaxy
sample represents the final set of galaxies observed using the original SDSS
target selection criteria. We analyse the clustering of galaxies within this
sample, including both the Luminous Red Galaxy (LRG) and Main samples, and also
include the 2-degree Field Galaxy Redshift Survey (2dFGRS) data. Baryon
Acoustic Oscillations are observed in power spectra measured for different
slices in redshift; this allows us to constrain the distance--redshift relation
at multiple epochs. We achieve a distance measure at redshift z=0.275, of
r_s(z_d)/D_V(0.275)=0.1390+/-0.0037 (2.7% accuracy), where r_s(z_d) is the
comoving sound horizon at the baryon drag epoch,
D_V(z)=[(1+z)^2D_A^2cz/H(z)]^(1/3), D_A(z) is the angular diameter distance and
H(z) is the Hubble parameter. We find an almost independent constraint on the
ratio of distances D_V(0.35)/D_V(0.2)=1.736+/-0.065, which is consistent at the
1.1sigma level with the best fit Lambda-CDM model obtained when combining our
z=0.275 distance constraint with the WMAP 5-year data. The offset is similar to
that found in previous analyses of the SDSS DR5 sample, but the discrepancy is
now of lower significance, a change caused by a revised error analysis and a
change in the methodology adopted, as well as the addition of more data. Using
WMAP5 constraints on Omega_bh^2 and Omega_ch^2, and combining our BAO distance
measurements with those from the Union Supernova sample, places a tight
constraint on Omega_m=0.286+/-0.018 and H_0 = 68.2+/-2.2km/s/Mpc that is robust
to allowing curvature and non-Lambda dark energy. This result is independent of
the behaviour of dark energy at redshifts greater than those probed by the BAO
and supernova measurements. (abridged)Comment: 22 pages, 16 figures, minor changes to match version published in
MNRA