323 research outputs found
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
Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)
Seven of ten candidate H-alpha emission-line stars found in an objective
grism survey of a 1 square degree region in MBM 18, were observed
spectroscopically. Four of these have weak H-alpha emission, and 6 out of 7
have spectral types M1-M4V. One star is of type F7-G1V, and has H-alpha in
absorption. The spectra of three of the M-stars may show an absorption line of
LiI, although none of these is an unambiguous detection. For the six M-stars a
good fit is obtained with pre-main-sequence isochrones indicating ages between
7.5 and 15Myr. The molecular cloud mass, derived from the integrated 12CO(1-0)
emission, is 160Mo (for a distance of 120pc), much smaller than the virial mass
(10^3Mo), and the cloud is not gravitationally bound. Nor are the individual
clumps we identified through a clump-finding routine. Considering the relative
weakness or absence of the H-alpha emission, the absence of other emission
lines, and the lack of clear LiI absorption, the targets are not T Tauri stars.
With ages between 7.5 and 15Myr they are old enough to explain the lack of
lithium in their spectra. Based on the derived distances (60-250pc), some of
the stars may lie inside the molecular cloud (120-150pc). From the fact that
the cloud as a whole, as well as the individual clumps, are not gravitationally
bound, in combination with the ages of the stars we conclude that it is not
likely that (these) stars were formed in MBM 18.Comment: Accepted for publication in Astronomy & Astrophysics (20 pages
The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter
We use radio-wave scattering data to constrain the distribution of ionized
gas in the outer Galaxy. Like previous models, our model for the H II disk
includes parameters for the radial scale length and scale height of the H II,
but we allow the H II disk to warp and flare. Our model also includes the
Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7
pulsars. Scattering in the Perseus arm is no more than 60% of the level
contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz
scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring
disk with a 1 kpc scale height, though this may reflect the non-uniform and
coarse coverage provided by the available data. The lack of a warp indicates
that VLBI observations near 1 GHz with an orbiting station having baseline
lengths of a few Earth diameters will not be affected by interstellar
scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is
15--20 kpc, but the data cannot distinguish between a gradual decrease in the
electron density and a truncated distribution. We favor a truncated one,
because we associate the scattering with massive star formation, which is also
truncated near 20 kpc. The distribution of electron density turbulence
decreases more rapidly with Galactocentric distance than does the hydrogen
distribution. Alternate ionizing and turbulent agents---the intergalactic
ionizing flux and satellite galaxies passing through the disk---do not
contribute significantly to scattering. We cannot exclude the possibility that
a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that
for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript
files, accepted for publication in Ap
Clumpy outer Galaxy molecular clouds and the steepening of the IMF
We report the results of high-resolution (~0.2 pc) CO(1-0) and CS(2-1)
observations of the central regions of three star-forming molecular clouds in
the far-outer Galaxy (~16 kpc from the Galactic Center): WB89 85 (Sh 2-127),
WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m
and NRAO 12-m observations. The GMC's in which the regions are embedded were
studied by means of KOSMA 3-m CO(2-1) observations. The properties the CO and
CS clumps are analyzed and compared with newly derived results of previously
published single-dish measurements of local clouds (OrionB South and Rosette).
We find that the slopes of the clump mass distributions (-1.28 and -1.49, for
WB89 85 and WB89 380, respectively) are somewhat less steep than found for most
local clouds, but similar to those of clouds which have been analyzed with the
same clumpfind program. We investigate the clump stability by using the virial
theorem, including all possible contributions (gravity, turbulence, magnetic
fields, and pressure due to the interclump gas). It appears that under
reasonable assumptions a combination of these forces would render most clumps
stable. Comparing only gravity and turbulence, we find that in the far-outer
Galaxy clouds, these forces are in equilibium (virial parameter alpha~1) for
clumps down to the lowest masses found (a few Msol). For clumps in the local
clouds alpha~1 only for clumps with masses larger than a few tens of Msol. Thus
it appears that in these outer Galaxy clumps gravity is the dominant force down
to a much lower mass than in local clouds, implying that gravitational collapse
and star formation may occur more readily even in the smallest clumps. Although
there are some caveats, due to the inhomogeneity of the data used, this might
explain the apparently steeper IMF found in the outer Galaxy.Comment: 29 pages, including 9 tables, 21 figures. Accepted for Astron.
Astrop
A star cluster at the edge of the Galaxy
We study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789) through NIR (JHK), molecular line-, and dust continuum observations. The kinematic distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of about 20.2 kpc, this object is at the edge of the (molecular) disk of the Galaxy. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ca. 1.3 pc and an average stellar surface density of ca. 12 pc^{-2}. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded in a 1000 Mo molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is < 40 K, and the total mass of the swept-up material is < 10 Mo. Near the center of the flow, indications of much higher temperatures are found, probably due to shocks. A spectrum of one of the probable cluster members shows a tentative likeness to that of a K3III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud
The distance to a star forming region in the Outer arm of the Galaxy
We performed astrometric observations with the VLBA of WB89-437, an H2O maser
source in the Outer spiral arm of the Galaxy. We measure an annual parallax of
0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2
kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the
heliocentric distance is considerably smaller than the kinematic distance of
8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135
degrees. We also measured the full space motion of the object and find a large
peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion
explains the large error in the kinematic distance estimate. We also find that
WB89-437 has the same rotation speed as the LSR, providing more evidence for a
flat rotation curve and thus the presence of dark matter in the outer Galaxy.Comment: The Astrophysical Journal, accepted, 16 pages, 4 Figure
The occultation events of the Herbig Ae/Be star V1247 Ori
Aims: I study new deep (DeltaV ~ 1.20-1.65 mag) occultation events of the
delta Scuti, Herbig Ae/Be star V1247 Ori in the Ori OB1 b association. Methods:
I use the V-band ASAS light curve of V1247 Ori, which covers the last nine
years, together with photometric data in the near-ultraviolet, visible, near-,
and far-infrared taken from the literature. I carry out a periodogram analysis
of the "cleaned" light curve and construct the spectral energy distribution of
the star. Results: The star V1247 Ori is interesting for the study of the UX
Orionis phenomenon, in which Herbig Ae/Be stars are occulted by their
protoplanetary discs, for three reasons: brightness (V ~ 9.85 mag), large
infrared excess at 20-100 mum (F_60 ~ 10 Jy), and photometric stability out of
occultation (sigma(V) ~ 0.02 mag), which may help to determine the location and
spatial structure of the occulting disc clumps.Comment: A&A Letters, in pres
Star Formation in the Most Distant Molecular Cloud in the Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of the Galaxy's Formation
We report the discovery of active star formation in Digel's Cloud 2, which is
one of the most distant giant molecular clouds known in the extreme outer
Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite
different environment from that in the solar neighborhood, including lower
metallicity, much lower gas density, and small or no perturbation from spiral
arms. With new wide-field near-infrared (NIR) imaging that covers the entire
Cloud 2, we discovered two young embedded star clusters located in the two
dense cores of the cloud. Using our NIR and 12CO data as well as HI, radio
continuum, and IRAS data in the archives, we discuss the detailed star
formation processes in this unique environment. We show clear evidences of a
sequential star formation triggered by the nearby huge supernova remnant, GSH
138-01-94. The two embedded clusters show a distinct morphology difference: the
one in the northern molecular cloud core is a loose association with
isolated-mode star formation, while the other in the southern molecular cloud
core is a dense cluster with cluster-mode star formation. We propose that high
compression by the combination of the SNR shell and an adjacent shell caused
the dense cluster formation in the southern core. Along with the low
metallicity range of the EOG, we suggest that EOG could be an excellent
laboratory for the study of star formation processes, such as those triggered
by supernovae, that occured during an early epoch of the Galaxy's formation. In
particular, the study of the EOG may shed light on the origin and role of the
thick disk, whose metallicity range matches with that of the EOG well.Comment: Accepted by The Astrophysical Journal (18 pages, 9 figures; a version
w/full-resolution color figures is available at
http://www.ioa.s.u-tokyo.ac.jp/~naoto/papers/apj.cl2_quirc/ms2p_final.pdf
Galactic interstellar 18O/17O ratios - a radial gradient?
(Abridged) Our aim is to determine 18O/17O abundance ratios across the entire
Galaxy. These provide a measure of the amount of enrichment by high-mass versus
intermediate-mass stars. Such ratios, derived from the C18O and C17O J=1-0
lines alone, may be affected by systematic errors. Therefore, the C18O and C17O
(1-0), (2-1), and (3-2), as well as the 13CO (1-0) and (2-1) lines, were
observed towards 18 prominent galactic targets (a total of 25 positions). The
combined dataset was analysed with an LVG model, accounting for optical depth
effects. The data cover galactocentric radii R between 0.1 and 16.9 kpc (solar
circle at 8.5 kpc). Near the centre of the Galaxy, 18O/17O = 2.88 +/- 0.11. For
the galactic disc out to an R of ca. 10 kpc, 18O/17O = 4.16 +/- 0.09. At ca. R
= 16.5 kpc, 18O/17O = 5.03 +/- 0.46. Assuming that 18O is synthesised
predominantly in high-mass stars (M > 8 Msun), while C17O is mainly a product
of lower-mass stars, the ratio from the inner Galaxy indicates a dominance of
CNO-hydrogen burning products that is also apparent in the C- and N-isotope
ratios. The high 18O/17O value of the solar system (5.5) relative to that of
the ambient ISM suggests contamination by nearby high-mass stars during its
formation. High values in the metal-poor environment of the outer Galaxy are
not matched by the low values observed towards the even more metal-poor LMC.
Apparently, the outer Galaxy cannot be considered as an intermediate
environment between the solar neighbourhood and the ISM of small metal-poor
galaxies. The apparent 18O/17O gradient along the galactic disc and the
discrepancy between outer disc and LMC isotope ratios may be explained by
different ages of the respective stellar populations.Comment: Accepted by Astron. & Astroph.; 10 pages + 4 pages on-line material
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