13 research outputs found
An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328
Galactic B and late-O stars. We use a new technique which employs stellar
atmosphere models in lieu of unreddened "standard" stars. This technique is
capable of virtually eliminating spectral mismatch errors in the curves. It
also allows a quantitative assessment of the errors and enables a rigorous
testing of the significance of relationships between various curve parameters,
regardless of whether their uncertainties are correlated. Analysis of the
curves gives the following results: (1) In accord with our previous findings,
the central position of the 2175 A extinction bump is mildly variable, its
width is highly variable, and the two variations are unrelated. (2) Strong
correlations are found among some extinction properties within the UV region,
and within the IR region. (3) With the exception of a few curves with extreme
(i.e., large) values of R(V), the UV and IR portions of Galactic extinction
curves are not correlated with each other. (4) The large sightline-to-sightline
variation seen in our sample implies that any average Galactic extinction curve
will always reflect the biases of its parent sample. (5) The use of an average
curve to deredden a spectral energy distribution (SED) will result in
significant errors, and a realistic error budget for the dereddened SED must
include the observed variance of Galactic curves. While the observed large
sightline-to-sightline variations, and the lack of correlation among the
various features of the curves, make it difficult to meaningfully characterize
average extinction properties, they demonstrate that extinction curves respond
sensitively to local conditions. Thus, each curve contains potentially unique
information about the grains along its sightline.Comment: To appear in the Astrophysical Journal, Part 1, July 1, 2007. Figures
and Tables which will appear only in the electronic version of the Journal
can be obtained via anonymous ftp from ftp://ftp.astronomy.villanova.edu .
After logging in, change directories to "fitz/FMV_EXTINCTION". A README file
describes the various files present in the director
The Palomar Testbed Interferometer Calibrator Catalog
The Palomar Testbed Interferometer (PTI) archive of observations between 1998
and 2005 is examined for objects appropriate for calibration of optical
long-baseline interferometer observations - stars that are predictably
point-like and single. Approximately 1,400 nights of data on 1,800 objects were
examined for this investigation. We compare those observations to an
intensively studied object that is a suitable calibrator, HD217014, and
statistically compare each candidate calibrator to that object by computing
both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis
is that the frequency distribution of visibility data associated with
calibrator stars differs from non-calibrator stars such as binary stars.
Spectroscopic binaries resolved by PTI, objects known to be unsuitable for
calibrator use, are similarly tested to establish detection limits of this
approach. From this investigation, we find more than 350 observed stars
suitable for use as calibrators (with an additional being
rejected), corresponding to sky coverage for PTI. This approach
is noteworthy in that it rigorously establishes calibration sources through a
traceable, empirical methodology, leveraging the predictions of spectral energy
distribution modeling but also verifying it with the rich body of PTI's on-sky
observations.Comment: 100 pages, 7 figures, 7 tables; to appear in the May 2008ApJS, v176n
Spitzer Observations of Bok Globule B335: Isolated Star Formation Efficiency and Cloud Structure
We present infrared and millimeter observations of Barnard 335, the
prototypical isolated Bok globule with an embedded protostar. Using Spitzer
data we measure the source luminosity accurately; we also constrain the density
profile of the innermost globule material near the protostar using the
observation of an 8.0 um shadow. HHT observations of 12CO 2 --> 1 confirm the
detection of a flattened molecular core with diameter ~10000 AU and the same
orientation as the circumstellar disk (~100 to 200 AU in diameter). This
structure is probably the same as that generating the 8.0 um shadow and is
expected from theoretical simulations of collapsing embedded protostars. We
estimate the mass of the protostar to be only ~5% of the mass of the parent
globule.Comment: 15 pages, 17 figures, emulateapj format, accepted for publication in
Ap