116 research outputs found
Methods for Estimating Fluxes and Absorptions of Faint X-ray Sources
X-ray sources with very few counts can be identified with low-noise X-ray
detectors such as ACIS onboard the Chandra X-ray Observatory. These sources are
often too faint for parametric spectral modeling using well-established methods
such as fitting with XSPEC. We discuss the estimation of apparent and intrinsic
broad-band X-ray fluxes and soft X-ray absorption from gas along the line of
sight to these sources, using nonparametric methods. Apparent flux is estimated
from the ratio of the source count rate to the instrumental effective area
averaged over the chosen band. Absorption, intrinsic flux, and errors on these
quantities are estimated from comparison of source photometric quantities with
those of high S/N spectra that were simulated using spectral models
characteristic of the class of astrophysical sources under study. The concept
of this method is similar to the long-standing use of color-magnitude diagrams
in optical and infrared astronomy, with X-ray median energy replacing color
index and X-ray source counts replacing magnitude. Our nonparametric method is
tested against the apparent spectra of 2000 faint sources in the Chandra
observation of the rich young stellar cluster in the M17 HII region. We show
that the intrinsic X-ray properties can be determined with little bias and
reasonable accuracy using these observable photometric quantities without
employing often uncertain and time-consuming methods of non-linear parametric
spectral modeling. Our method is calibrated for thermal spectra characteristic
of stars in young stellar clusters, but recalibration should be possible for
some other classes of faint X-ray sources such as extragalactic AGN.Comment: Accepted for publication in The Astrophysical Journal. 39 pages, 15
figure
The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region
Rich young stellar clusters produce HII regions whose expansion into the
nearby molecular cloud is thought to trigger the formation of new stars.
However, the importance of this mode of star formation is uncertain. This
investigation seeks to quantify triggered star formation (TSF) in IC 1396A
(a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the
periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37
cluster. X-ray selection of young stars from Chandra X-ray Observatory data is
combined with existing optical and infrared surveys to give a more complete
census of the TSF population. Over 250 young stars in and around IC 1396A are
identified; this doubles the previously known population. A spatio-temporal
gradient of stars from the IC 1396A cloud toward the primary ionizing star HD
206267 is found. We argue that the TSF mechanism in IC 1396A is the
radiation-driven implosion process persisting over several million years.
Analysis of the X-ray luminosity and initial mass functions indicates that >140
stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC
1396 HII region, we estimate the TSF contribution for the entire HII region
exceeds 14-25% today, and may be higher over the lifetime of the HII region.
Such triggering on the periphery of HII regions may be a significant mode of
star formation in the Galaxy.Comment: Accepted for publication in MNRAS; 28 pages, 18 figure
A Naive Bayes Source Classifier for X-ray Sources
The Chandra Carina Complex Project (CCCP) provides a sensitive X-ray survey
of a nearby starburst region over >1 square degree in extent. Thousands of
faint X-ray sources are found, many concentrated into rich young stellar
clusters. However, significant contamination from unrelated Galactic and
extragalactic sources is present in the X-ray catalog. We describe the use of a
naive Bayes classifier to assign membership probabilities to individual
sources, based on source location, X-ray properties, and visual/infrared
properties. For the particular membership decision rule adopted, 75% of CCCP
sources are classified as members, 11% are classified as contaminants, and 14%
remain unclassified. The resulting sample of stars likely to be Carina members
is used in several other studies, which appear in a Special Issue of the ApJS
devoted to the CCCP.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
least. 19 pages, 7 figure
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