416 research outputs found
Photometric Redshifts and Photometry Errors
We examine the impact of non-Gaussian photometry errors on photometric
redshift performance. We find that they greatly increase the scatter, but this
can be mitigated to some extent by incorporating the correct noise model into
the photometric redshift estimation process. However, the remaining scatter is
still equivalent to that of a much shallower survey with Gaussian photometry
errors. We also estimate the impact of non-Gaussian errors on the spectroscopic
sample size required to verify the photometric redshift rms scatter to a given
precision. Even with Gaussian {\it photometry} errors, photometric redshift
errors are sufficiently non-Gaussian to require an order of magnitude larger
sample than simple Gaussian statistics would indicate. The requirements
increase from this baseline if non-Gaussian photometry errors are included.
Again the impact can be mitigated by incorporating the correct noise model, but
only to the equivalent of a survey with much larger Gaussian photometry errors.
However, these requirements may well be overestimates because they are based on
a need to know the rms, which is particularly sensitive to tails. Other
parametrizations of the distribution may require smaller samples.Comment: submitted to ApJ
Color-Induced Displacement double stars in SDSS
We report the first successful application of the astrometric color-induced
displacement technique (CID, the displacement of the photocenter between
different bandpasses due to a varying contribution of differently colored
components to the total light), originally proposed by Wielen (1996) for
discovering unresolved binary stars. Using the Sloan Digital Sky Survey (SDSS)
Data Release 1 with 2.5 million stars brighter than 21m in the u and g bands,
we select 419 candidate binary stars with CID greater than 0.5 arcsec. The SDSS
colors of the majority of these candidates are consistent with binary systems
including a white dwarf and any main sequence star with spectral type later
than ~K7. The astrometric CID method discussed here is complementary to the
photometric selection of binary stars in SDSS discussed by Smolcic et al.
(2004), but there is considerable overlap (15%) between the two samples of
selected candidates. This overlap testifies both to the physical soundness of
both methods, as well as to the astrometric and photometric quality of SDSS
data.Comment: submitted to A&A, 13 pages, 6 figure
3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium
We use 3-D radiative transfer models to show the effects of clumpy
circumstellar material on the observed infrared colors of high mass stars
embedded in molecular clouds. We highlight differences between 3-D clumpy and
1-D smooth models which can affect the interpretation of data. We discuss
several important properties of the emergent spectral energy distribution
(SED): More near-infrared light (scattered and direct from the central source)
can escape than in smooth 1-D models. The near- and mid-infrared SED of the
same object can vary significantly with viewing angle, depending on the clump
geometry along the sightline. Even the wavelength-integrated flux can vary with
angle by more than a factor of two. Objects with the same average circumstellar
dust distribution can have very different near-and mid-IR SEDs depending on the
clump geometry and the proximity of the most massive clump to the central
source.
Although clumpiness can cause similar objects to have very different SEDs,
there are some observable trends. Near- and mid-infrared colors are sensitive
to the weighted average distance of clumps from the central source and to the
magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared
emission remains a robust measure of the total dust mass. We present simulated
SEDs, colors, and images for 2MASS and Spitzer filters. We compare to
observations of some UCHII regions and find that 3-D clumpy models fit better
than smooth models. In particular, clumpy models with fractal dimensions in the
range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with
shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures:
http://www.astro.virginia.edu/~ri3e/clumpy3d.pd
The Mid-Infrared Emitting Dust Around AB Aur
Using the Keck I telescope, we have obtained 11.7 micron and 18.7 micron
images of the circumstellar dust emission from AB Aur, a Herbig Ae star. We
find that AB Aur is probably resolved at 18.7 micron with an angular diameter
of 1.2" at a surface brightness of 3.5 Jy/arcsec^2. Most of the dust mass
detected at millimeter wavelengths does not contribute to the 18.7 micron
emission, which is plausibly explained if the system possesses a relatively
cold, massive disk. We find that models with an optically thick, geometrically
thin disk, surrounded by an optically thin spherical envelope fit the data
somewhat better than flared disk models.Comment: ApJ in press, 4 color figure
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
Relation between the luminosity of young stellar objects and their circumstellar environment
We present a new model-independent method of comparison of NIR visibility
data of YSOs. The method is based on scaling the measured baseline with the
YSO's distance and luminosity, which removes the dependence of visibility on
these two variables. We use this method to compare all available NIR visibility
data and demonstrate that it distinguishes YSOs of luminosity >1000L_sun
(low-L) from YSOs of <1000L_sun (high-L). This confirms earlier suggestions,
based on fits of image models to the visibility data, for the difference
between the NIR sizes of these two luminosity groups. When plotted against the
``scaled'' baseline, the visibility creates the following data clusters: low-L
Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. The T Tau
cluster is similar to the low-L Herbig Ae/Be cluster, which has ~7 times
smaller ``scaled'' baselines than the high-L Herbig Be cluster. We model the
shape and size of clusters with different image models and find that low-L
Herbig stars are the best explained by the uniform brightness ring and the halo
model, T Tauri stars with the halo model, and high-L Herbig stars with the
accretion disk model. However, the plausibility of each model is not well
established. Therefore, we try to build a descriptive model of the
circumstellar environment consistent with various observed properties of YSOs.
We argue that low-L YSOs have optically thick disks with the optically thin
inner dust sublimation cavity and an optically thin dusty outflow above the
inner disk regions. High-L YSOs have optically thick accretion disks with high
accretion rates enabling gas to dominate the NIR emission over dust. Although
observations would favor such a description of YSOs, the required dust
distribution is not supported by our current understanding of dust dynamics.Comment: 20 pages, 12 figures, Accepted for publication in the Astrophysical
Journa
2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources
We present 2-D radiation transfer models of Class I Protostars and show the
effect of including more realistic geometries on the resulting spectral energy
distributions and images. We begin with a rotationally flattened infalling
envelope as our comparison model, and add a flared disk and bipolar cavity. The
disk affects the spectral energy distribution most strongly at edge-on
inclinations, causing a broad dip at about 10 um (independent of the silicate
feature) due to high extinction and low scattering albedo in this wavelength
region. The bipolar cavities allow more direct stellar+disk radiation to emerge
into polar directions, and more scattering radiation to emerge into all
directions. The wavelength-integrated flux, often interpreted as luminosity,
varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much
flux as edge-on, depending on geometry. Thus, observational estimates of
luminosity should take into account the inclination of a source. The envelopes
with cavities are significantly bluer in near-IR and mid-IR color-color plots
than those without cavities. Using 1-D models to interpret Class I sources with
bipolar cavities would lead to an underestimate of envelope mass and an
overestimate of the implied evolutionary state. We compute images at near-,
mid-, and far-IR wavelengths. We find that the mid-IR colors and images are
sensitive to scattering albedo, and that the flared disk shadows the midplane
on large size scales at all wavelengths plotted. Finally, our models produce
polarization spectra which can be used to diagnose dust properties, such as
albedo variations due to grain growth. Our results of polarization across the
3.1 um ice feature agree well with observations for ice mantles covering 5% of
the radius of the grains.Comment: Accepted for publication in ApJ, 37 pages, 13 figures (several
figures reduced in quality; find original version at
http://gemelli.colorado.edu/~bwhitney/preprints.html
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