141 research outputs found
Dusty OB stars in the Small Magellanic Cloud - II: Extragalactic Disks or Examples of the Pleiades Phenomenon?
We use mid-infrared Spitzer spectroscopy and far-infrared Herschel photometry
for a sample of twenty main sequence O9--B2 stars in the Small Magellanic Cloud
(SMC) with strong 24 micron excesses to investigate the origin of the mid-IR
emission. Either debris disks around the stars or illuminated patches of dense
interstellar medium (ISM) can cause such mid-IR emission. In a companion paper,
Paper I, we use optical spectroscopy to show that it is unlikely for any of
these sources to be classical Be stars or Herbig Ae/Be stars. We focus our
analysis on debris disks and cirrus hot spots. We find three out of twenty
stars to be significantly extended in the mid-IR, establishing them as cirrus
hot spots. We then fit the IR spectral energy distributions to determine dust
temperatures and masses. We find the dust masses in the SMC stars to be larger
than for any known debris disks, although this evidence against the debris disk
hypothesis is circumstantial. Finally, we created a local comparison sample of
bright mid-IR OB stars in the Milky Way (MW) by cross-matching the WISE and
Hipparcos catalogs. All such local stars in the appropriate luminosity range
that can be unambiguously classified are young stars with optical emission
lines or are spatially resolved by WISE with sizes too large to be plausible
debris disk candidates. We conclude that the very strong mid-IR flux excesses
are most likely explained as cirrus hot spots, although we cannot rigorously
rule out that a small fraction of the sample is made up of debris disks or
transition disks. We present suggestive evidence that bow-shock heating around
runaway stars may be a contributing mechanism to the interstellar emission.
These sources, interpreted as cirrus hot spots, offer a new localised probe of
diffuse interstellar dust in a low metallicity environment. (Abridged)Comment: Accepted for publication in ApJ, 23 pages, 11 figures, 8 table
A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
We determine the parallax and proper motion of the flaring, non-thermal radio
star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline
Array observations. Based on the parallax, we measure a distance of 389 +24/-21
parsecs to the source. Our measurement places the Orion Nebula Cluster
considerably closer than the canonical distance of 480 +/- 80 parsecs
determined by Genzel et al. (1981). A change of this magnitude in distance
lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We
briefly discuss two effects of this change--an increase in the age spread of
the pre-main sequence stars and better agreement between the zero-age
main-sequence and the temperatures and luminosities of massive stars.Comment: 10 pages, 4 figures, emulateapj, accepted to Ap
The Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE): The Dust Extinction Curve from Red Clump Stars
We use Hubble Space Telescope (HST) observations of red clump stars taken as
part of the Small Magellanic Cloud Investigation of Dust and Gas Evolution
(SMIDGE) program to measure the average dust extinction curve in a ~ 200 pc x
100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). The
rich information provided by our 8-band ultra-violet through near-infrared
photometry allows us to model the color-magnitude diagram of the red clump
accounting for the extinction curve shape, a log-normal distribution of
, and the depth of the stellar distribution along the line of sight. We
measure an extinction curve with = 2.65
0.11. This measurement is significantly larger than the equivalent values
of published Milky Way = 3.1 () and SMC Bar =
2.74 () extinction curves. Similar extinction curve offsets in
the Large Magellanic Cloud (LMC) have been interpreted as the effect of large
dust grains. We demonstrate that the line-of-sight depth of the SMC (and LMC)
introduces an apparent "gray" contribution to the extinction curve inferred
from the morphology of the red clump. We show that no gray dust component is
needed to explain extinction curve measurements when a full-width half-max
depth of 10 2 kpc in the stellar distribution of the SMC (5 1 kpc
for the LMC) is considered, which agrees with recent studies of Magellanic
Cloud stellar structure. The results of our work demonstrate the power of
broad-band HST imaging for simultaneously constraining dust and galactic
structure outside the Milky Way.Comment: 16 pages, 12 figures, 5 tables. Accepted for publication in Ap
Keck Cosmic Web Imager Observations of He II Emission in I Zw 18
With a metallicity of 12 + Log(O/H) ≍ 7.1-7.2, I Zw 18 is a canonical low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including I Zw 18, have been found to host strong, narrow-lined, nebular He II (λ4686) emission with enhanced intensities compared to Hβ (e.g., He II(λ4686)/Hβ > 1%). We present new observations of I Zw 18 using the Keck Cosmic Web Imager. These observations reveal two nebular He II emission regions (or He III regions) northwest and southeast of the He III region in the galaxy's main body investigated in previous studies. All regions exhibit He II(λ4686)/Hβ greater than 2%. The two newly resolved He III regions lie along an axis that intercepts the position of I Zw 18's ultraluminous X-ray (ULX) source. We explore whether the ULX could power the two He III regions via shock activity and/or beamed X-ray emission. We find no evidence of shocks from the gas kinematics. If the ULX powers the two regions, the X-ray emission would need to be beamed. Another potential explanation is that a class of early-type nitrogen-rich Wolf-Rayet stars with low winds could power the two He III regions, in which case the alignment with the ULX would be coincidental
The PHANGS-JWST Treasury Survey: Star Formation, Feedback, and Dust Physics at High Angular Resolution in Nearby GalaxieS
The PHANGS collaboration has been building a reference data set for the multiscale, multiphase study of star formation and the interstellar medium (ISM) in nearby galaxies. With the successful launch and commissioning of JWST, we can now obtain high-resolution infrared imaging to probe the youngest stellar populations and dust emission on the scales of star clusters and molecular clouds (∼5-50 pc). In Cycle 1, PHANGS is conducting an eight-band imaging survey from 2 to 21 μm of 19 nearby spiral galaxies. Optical integral field spectroscopy, CO(2-1) mapping, and UV-optical imaging for all 19 galaxies have been obtained through large programs with ALMA, VLT-MUSE, and Hubble. PHANGS-JWST enables a full inventory of star formation, accurate measurement of the mass and age of star clusters, identification of the youngest embedded stellar populations, and characterization of the physical state of small dust grains. When combined with Hubble catalogs of ∼10,000 star clusters, MUSE spectroscopic mapping of ∼20,000 H ii regions, and ∼12,000 ALMA-identified molecular clouds, it becomes possible to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity, all across a diversity of galactic environments. Here we describe the PHANGS-JWST Treasury survey, present the remarkable imaging obtained in the first few months of science operations, and provide context for the initial results presented in the first series of PHANGS-JWST publications
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