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 $r^{-2}$, while young objects have profiles that vary as $r^{-3/2}$ 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 $1/r$ or $1/r^2$. 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