24 research outputs found
The Frequency of Warm Debris Disks and Transition Disks in a Complete Sample of Intermediate-Mass GLIMPSE Stars: Placing Constraints on Disk Lifetimes
The incidence of dusty debris disks around low- and intermediate-mass stars
has been investigated numerous times in order to understand the early stages of
planet formation. Most notably, the IRAS mission observed the entire sky at
mid- and far-IR wavelengths, identifying the first debris disk systems, but was
unable to detect a statistically significant sample of warm debris disks due to
its limited sensitivity at 12 microns. Using Tycho-2 Spectral Catalog stars
previously shown to exhibit 8 micron mid-infrared circumstellar excesses
confirmed at 24 microns in the Spitzer GLIMPSE survey, we investigate the
frequency of mid-IR excesses among intermediate-mass (2--4 solar mass) stars in
a complete volume-limited sample. Our study of 338 stars is four times larger
than a complete sample of 12 micron sources from the IRAS Point Source Catalog.
We find that 0.3+-0.3% of intermediate-mass stars exhibit a signature of a
possible terrestrial-temperature debris disks at wavelengths of 8 microns and
greater. We also find that 1.2+-0.6% of intermediate-mass stars exhibit
evidence for circumstellar disks undergoing inner disk clearing, i.e.,
candidate transition disk systems. Using stellar lifetimes and the frequency of
transition and primordial disks within a given spectral type, we find that
pre-main-sequence disks around intermediate-mass stars dissipate in 5+-2 Myr,
consistent with other studies.Comment: 4 Figures. Accepted in the Astronomical Journa
The Bubbling Galactic Disk
A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H II regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H II regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H II regions. Some of the bubbles may be the outer edges of H II regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1' and 3' with over 98% having angular diameters less than 10' and 88% less than 4'. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85
A Sample of Intermediate-Mass Star-Forming Regions: Making Stars at Mass Column Densities <1 g/cm^2
In an effort to understand the factors that govern the transition from low-
to high-mass star formation, we identify for the first time a sample of
intermediate-mass star-forming regions (IM SFRs) where stars up to - but not
exceeding - 8 solar masses are being produced. We use IRAS colors and Spitzer
Space Telescope mid-IR images, in conjunction with millimeter continuum and CO
maps, to compile a sample of 50 IM SFRs in the inner Galaxy. These are likely
to be precursors to Herbig AeBe stars and their associated clusters of low-mass
stars. IM SFRs constitute embedded clusters at an early evolutionary stage akin
to compact HII regions, but they lack the massive ionizing central star(s). The
photodissociation regions that demarcate IM SFRs have typical diameters of ~1
pc and luminosities of ~10^4 solar luminosities, making them an order of
magnitude less luminous than (ultra)compact HII regions. IM SFRs coincide with
molecular clumps of mass ~10^3 solar masses which, in turn, lie within larger
molecular clouds spanning the lower end of the giant molecular cloud mass
range, 10^4-10^5 solar masses. The IR luminosity and associated molecular mass
of IM SFRs are correlated, consistent with the known luminosity-mass
relationship of compact HII regions. Peak mass column densities within IM SFRs
are ~0.1-0.5 g/cm^2, a factor of several lower than ultra-compact HII regions,
supporting the proposition that there is a threshold for massive star formation
at ~1 g/cm^2.Comment: 61 pages, 6 tables, 20 figures. Accepted for publication in the
Astronomical Journa
The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 micron
mid-infrared extraphotospheric excesses in the MidCourse Space Experiment (MSX)
and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire
(GLIMPSE) surveys. Of these, 183 are either OB stars earlier than B8 in which
the excess plausibly arises from a thermal bremsstrahlung component or evolved
stars in which the excess may be explained by an atmospheric dust component.
The remaining 47 stars have spectral classifications B8 or later and appear to
be main sequence or late pre-main-sequence objects harboring circumstellar
disks. Six of the 47 stars exhibit multiple signatures characteristic of
pre-main-sequence circumstellar disks, including emission lines, near-infrared
K-band excesses, and X-ray emission. Approximately one-third of the remaining
41 sources have emission lines suggesting relative youth. Of the 25 GLIMPSE
stars with SST data at >24 microns, 20 also show an excess at 24 microns. Three
additional objects have 24 micron upper limits consistent with possible
excesses, and two objects have photospheric measurements at 24 microns. Six MSX
sources had a measurement at wavelengths >8 microns. We modeled the excesses in
26 stars having two or more measurements in excess of the expected photospheres
as single-component blackbodies. We determine probable disk temperatures and
fractional infrared luminosities in the range 191 < T < 787 and 3.9x10^-4 <
L_IR/L_* < 2.7x10^-1. We estimate a lower limit on the fraction of Tycho-2
Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses
to be 1.0+-0.3%.Comment: Accepted to Ap
A Radial Velocity Survey of the Cygnus OB2 Association
We conducted a radial velocity survey of the Cygnus OB2 Association over a 6
year (1999 - 2005) time interval to search for massive close binaries. During
this time we obtained 1139 spectra on 146 OB stars to measure mean systemic
radial velocities and radial velocity variations. We spectroscopically identify
73 new OB stars for the first time, the majority of which are likely to be
Association members. Spectroscopic evidence is also presented for a B3Iae
classification and temperature class variation (B3 - B8) on the order of 1 year
for Cygnus OB2 No. 12. Calculations of the intial mass function with the
current spectroscopic sample yield Gamma = -2.2 +/- 0.1. Of the 120 stars with
the most reliable data, 36 are probable and 9 are possible single-lined
spectroscopic binaries. We also identify 3 new and 8 candidate double-lined
spectroscopic binaries. These data imply a lower limit on the massive binary
fraction of 30% - 42%. The calculated velocity dispersion for Cygnus OB2 is
2.44 +/- km/s, which is typical of open clusters. No runaway OB stars were
found.Comment: 56 pages, 23 figures, 5 tables, accepted for publication in the
Astrophysical Journa
A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-mass stars
that exhibit an 8 micron excess in either the Galactic Legacy Infrared
Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high
resolution optical spectra to identify stars surrounded by warm circumstellar
dust. From these data we determine projected stellar rotational velocities and
estimate stellar effective temperatures for the sample. We estimate stellar
ages from these temperatures, parallactic distances, and evolutionary models.
Using MIPS [24] measurements and stellar parameters we determine the nature of
the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha
emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses
arise in circumstellar disks, we use the Halpha fluxes to model and estimate
the relative contributions of dust and free-free emission. Six stars exhibit
Halpha fluxes that imply free-free emission can plausibly explain the infrared
excess at [24]. These stars are candidate classical Be stars. Nine stars
exhibit Halpha emission, but their Halpha fluxes are insufficient to explain
the infrared excesses at [24], suggesting the presence of a circumstellar dust
component. After the removal of the free-free component in these sources, we
determine probable disk dust temperatures of Tdisk~300-800 K and fractional
infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be
pre-main-sequence stars with transitional disks undergoing disk clearing. Three
of the four sources showing Halpha absorption exhibit circumstellar disk
temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm
debris disk candidates. One of the four Halpha absorption sources has K-[24]>
3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap
A GLIMPSE of the Southern Jellyfish Nebula and Its Massive YSO
In Spitzer/IRAC images obtained under the GLIMPSE Legacy Survey, we have identified a unique and provocative nebular object we call the "Southern Jellyfish Nebula." The Southern Jellyfish Nebula is characterized by a fan of narrow tendrils with extreme length-to-width ratios that emanate from the vicinity of a bright infrared point source embedded in a smaller resolved nebula. From CO observations of the Nebula's morphologically associated molecular cloud, we have derived a kinematic distance of 5.7 ± 0.8 kpc and a cloud mass of 3.2 ± 0.9 × 10^3 M_⊙. The tendril-like ropes of the Nebula have widths of ~0.1 pc and lengths of up to ~2 pc. We have integrated the infrared spectral energy distribution (SED) of the point source to establish it as a massive young stellar object (MYSO), most likely forming alone, but possibly masking fainter cluster members. The shape of the SED is consistent with the shape of a late Class 0 SED model. Based on its far-IR luminosity of 3.3 ± 0.9 × 10^4 L_⊙, the Southern Jellyfish's MYSO has a zero-age main sequence (ZAMS) spectral type of B0. Given the curious nature of this nebula, we suspect its peculiar IR-bright structure is directly related to its current state of star formation
First GLIMPSE Results on the Stellar Structure of the Galaxy
The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source
Catalog of ~ 30 million mid-infrared sources towards the inner Galaxy, 10 < |l|
< 65 degrees and |b| < 1 degree, was used to determine the distribution of
stars in Galactic longitude, latitude, and apparent magnitude. The counts
versus longitude can be approximated by the modified Bessel function
N=N_0*(l/l_0)*K_1(l/l_0), where l_0 is insensitive to limiting magnitude, band
choice, and side of Galactic center: l_0= 17-30 degrees with a best fit value
in the the 4.5 micron band of l_0=24 +/- 4 degrees. Modeling the source
distribution as an exponential disk yields a radial scale length of H= 3.9 +/-
0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| <
30 degrees, with ~ 25% more stars in the north. For l=10-30 degrees, there is a
strong enhancement of stars of m= 11.5-13.5 mag. A linear bar passing through
the Galactic center with half-length R_bar=4.4 +/- 0.5 kpc, tilted by phi=44
+/- 10 degrees to the Sun-Galactic Center line, provides the simplest
interpretation of this data. We examine the possibility that enhanced source
counts at l=26-28 degrees, 31.5-34 degrees, and 306-309 degrees are related to
Galactic spiral structure. Total source counts are depressed in regions where
the counts of red objects (m_K-m_[8.0] >3) peak. In these areas, the counts are
reduced by extinction due to molecular gas and/or high diffuse backgrounds
associated with star formation.Comment: Accepted for publication in Astrophysical Journal Letters. Version
with full res figures and info on GLIMPSE Point Source Catalog at
http://www.astro.wisc.edu/glimps