936 research outputs found
Lunar base CELSS: A bioregenerative approach
During the twenty-first century, human habitation of a self-sustaining lunar base could become a reality. To achieve this goal, the occupants will have to have food, water, and an adequate atmosphere within a carefully designed environment. Advanced technology will be employed to support terrestrial life-sustaining processes on the Moon. One approach to a life support system based on food production, waste management and utilization, and product synthesis is outlined. Inputs include an atmosphere, water, plants, biodegradable substrates, and manufacutured materials such as fiberglass containment vessels from lunar resources. Outputs include purification of air and water, food, and hydrogen (H2) generated from methane (CH4). Important criteria are as follows: (1) minimize resupply from Earth; and (2) recycle as efficiently as possible
Mid-infrared interferometry of massive young stellar objects
The very inner structure of massive young stellar objects (YSOs) is difficult
to trace. With conventional observational methods we identify structures still
several hundreds of AU in size. However, the (proto-)stellar growth takes place
at the innermost regions (<100 AU) where the actual mass transfer onto the
forming high-mass star occurs. We present results from our programme toward
massive YSOs at the VLTI, utilising the two-element interferometer MIDI. To
date, we observed 10 well-known massive YSOs down to scales of 20 mas
(typically corresponding to 20 - 40 AU for our targets) in the 8-13 micron
region. We clearly resolve these objects which results in low visibilities and
sizes in the order of 30-50 mas. For two objects, we show results of our
modelling. We demonstrate that the MIDI data can reveal decisive structure
information for massive YSOs. They are often pivotal in order to resolve
ambiguities still immanent in model parameters derived from sole SED fitting.Comment: 6 pages, 5 figures, necessary style files iopams.sty, jpconf11.clo,
and jpconf.cls included; contribution for the conference "The Universe under
the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to
be published in Journal of Physics: Conference Series by Institute of Physics
Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HII Region NGC 346
We use Spitzer Space Telescope observations from the Spitzer Survey of the
Small Magellanic Cloud (S3MC) to study the young stellar content of N66, the
largest and brightest HII region in the SMC. In addition to large numbers of
normal stars, we detect a significant population of bright, red infrared
sources that we identify as likely to be young stellar objects (YSOs). We use
spectral energy distribution (SED) fits to classify objects as ordinary (main
sequence or red giant) stars, asymptotic giant branch stars, background
galaxies, and YSOs. This represents the first large-scale attempt at blind
source classification based on Spitzer SEDs in another galaxy. We firmly
identify at least 61 YSOs, with another 50 probable YSOs; only one embedded
protostar in the SMC was reported in the literature prior to the S3MC. We
present color selection criteria that can be used to identify a relatively
clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the
infrared-bright YSOs in N66 have stellar masses ranging from 2 Msun to 17 Msun,
and that approximately half of the objects are Stage II protostars, with the
remaining YSOs roughly evenly divided between Stage I and Stage III sources. We
find evidence for primordial mass segregation in the HII region, with the most
massive YSOs being preferentially closer to the center than lower-mass objects.
Despite the low metallicity and dust content of the SMC, the observable
properties of the YSOs appear consistent with those in the Milky Way. Although
the YSOs are heavily concentrated within the optically bright central region of
N66, there is ongoing star formation throughout the complex and we place a
lower limit on the star formation rate of 3.2 x 10^-3 Msun/yr over the last ~1
Myr.Comment: 13 pages, 5 figures (3 in color), 2 tables. Accepted for publication
in Ap
The Extended Environment of M17: A Star Formation History
M17 is one of the youngest and most massive nearby star-formation regions in
the Galaxy. It features a bright H II region erupting as a blister from the
side of a giant molecular cloud (GMC). Combining photometry from the Spitzer
GLIMPSE survey with complementary infrared (IR) surveys, we identify candidate
young stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes
the M17 complex. The long sightline through the Galaxy behind M17 creates
significant contamination in our YSO sample from unassociated sources with
similar IR colors. Removing contaminants, we produce a highly-reliable catalog
of 96 candidate YSOs with a high probability of association with the M17
complex. We fit model spectral energy distributions to these sources and
constrain their physical properties. Extrapolating the mass function of 62
intermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the
process of forming in the extended outer regions of M17.
From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim
of a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present
new maps of CO and 13CO (J=2-1) emission, which show that the shell is a
coherent, kinematic structure associated with M17 at v = 19 km/s. The shell is
an extended bubble outlining the photodissociation region of a faint, diffuse H
II region several Myr old. We provide evidence that massive star formation has
been triggered by the expansion of the bubble. The formation of the massive
cluster ionizing the M17 H II region itself may have been similarly triggered.
We conclude that the star formation history in the extended environment of M17
has been punctuated by successive waves of massive star formation propagating
through a GMC complex.Comment: 31 pages, 15 figures, accepted for publication in ApJ. For a version
with higher-quality figures, see
http://www.astro.wisc.edu/glimpse/Povich2009_M17.pd
Spiral arms and instability within the AFGL 4176 mm1 disc
We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama Large Millimeter/submillimeter Array observations at 1.2 mm of the disc around the forming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radius of ∼1000 au and contains significant structure, most notably a spiral arm on its redshifted side. We fitted the observed spiral with logarithmic and Archimedean spiral models. We find that both models can describe its structure, but the Archimedean spiral with a varying pitch angle fits its morphology marginally better. As well as signatures of rotation across the disc, we observe gas arcs in CH3CN that connect to other millimetre continuum sources in the field, supporting the picture of interactions within a small cluster around AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling of the CH3CN K-ladder, we determine the temperature and velocity field across the disc, and thus produce a map of the Toomre stability parameter. Our results indicate that the outer disc is gravitationally unstable and has already fragmented or is likely to fragment in the future, possibly producing further companions. These observations provide evidence that disc fragmentation is one possible pathway towards explaining the high fraction of multiple systems around high-mass stars
The youngest massive protostars in the Large Magellanic Cloud
We demonstrate the unique capabilities of Herschel to study very young
luminous extragalactic young stellar objects (YSOs) by analyzing a central
strip of the Large Magellanic Cloud obtained through the HERITAGE Science
Demonstration Program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500
microns photometry with 2MASS (1.25-2.17 microns) and Spitzer IRAC and MIPS
(3.6-70 microns) to construct complete spectral energy distributions (SEDs) of
compact sources. From these, we identify 207 candidate embedded YSOs in the
observed region, ~40% never-before identified. We discuss their position in
far-infrared color-magnitude space, comparing with previously studied,
spectroscopically confirmed YSOs and maser emission. All have red colors
indicating massive cool envelopes and great youth. We analyze four example
YSOs, determining their physical properties by fitting their SEDs with
radiative transfer models. Fitting full SEDs including the Herschel data
requires us to increase the size and mass of envelopes included in the models.
This implies higher accretion rates (greater than or equal to 0.0001 M_sun/yr),
in agreement with previous outflow studies of high-mass protostars. Our results
show that Herschel provides reliable longwave SEDs of large samples of
high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands;
and constrains the physical properties and evolutionary stages of YSOs more
precisely than was previously possible.Comment: Main text: 4 pages, 3 figures, 1 table; Online material: 3 figures, 1
table; to appear in the A&A Herschel Special Issu
Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major
We study a very young star-forming region in the outer Galaxy that is the
most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360
survey. This region, dubbed CMa-l224, is located in the Canis Major OB1
association. CMa-l224 is relatively faint in the mid-infrared, but it shines
brightly at the far-infrared wavelengths as revealed by the Herschel Space
Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 m data from
the Spitzer/GLIMPSE360 survey, combined with the JHK 2MASS and the 70-500
m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection
criteria based on color-color cuts and fitting of the YSO candidates' spectral
energy distributions with YSO 2D radiative transfer models. We identify 293 YSO
candidates and estimate physical parameters for 210 sources well-fit with YSO
models. We select an additional 47 sources with GLIMPSE360-only photometry as
`possible YSO candidates'. The vast majority of these sources are associated
with high H column density regions and are good targets for follow-up
studies. The distribution of YSO candidates at different evolutionary stages
with respect to Herschel filaments supports the idea that stars are formed in
the filaments and become more dispersed with time. Both the supernova-induced
and spontaneous star formation scenarios are plausible in the environmental
context of CMa-l224. However, our results indicate that a spontaneous
gravitational collapse of filaments is a more likely scenario. The methods
developed for CMa-l224 can be used for larger regions in the Galactic plane
where the same set of photometry is available.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 54 pages including appendice
Spitzer View of Young Massive Stars in the LMC HII Complex N44
The HII complex N44 in the Large Magellanic Cloud (LMC) provides an excellent
site to perform a detailed study of star formation in a mild starburst, as it
hosts three regions of star formation at different evolutionary stages and it
is not as complicated and confusing as the 30 Doradus giant HII region. We have
obtained Spitzer Space Telescope observations and complementary ground-based 4m
uBVIJK observations of N44 to identify candidate massive young stellar objects
(YSOs). We further classify the YSOs into Types I, II, and III, according to
their spectral energy distributions (SEDs). In our sample of 60 YSO candidates,
~65% of them are resolved into multiple components or extended sources in
high-resolution ground-based images. We have modeled the SEDs of 36 YSOs that
appear single or dominant within a group. We find good fits for Types I and
I/II YSOs,but Types II and II/III YSOs show deviations between their observed
SEDs and models that do not include PAH emission. We have also found that some
Type III YSOs have central holes in their disk components. YSO counterparts are
found in four ultracompact HII regions and their stellar masses determined from
SED model fits agree well with those estimated from the ionization requirements
of the HII regions. The distribution of YSOs is compared with those of the
underlying stellar population and interstellar gas conditions to illustrate a
correlation between the current formation of O-type stars and previous
formation of massive stars. Evidence of triggered star formation is also
presented.Comment: 63 pages, 15 figures, accepted for publication for ApJ; some figures
in jpeg format to meet the size limit; preprint with high resolution images
can be found at http://www.astro.virginia.edu/~cc5ye/n44yso.pd
Triggered Star Formation in a Double Shell near W51A
We present Heinrich Hertz Telescope CO observations of the shell structure
near the active star-forming complex W51A to investigate the process of star
formation triggered by the expansion of an HII region. The CO observations
confirm that dense molecular material has been collected along the shell
detected in Spitzer IRAC images. The CO distribution shows that the shell is
blown out toward a lower density region to the northwest. Total hydrogen column
density around the shell is high enough to form new stars. We find two CO
condensations with the same central velocity of 59 km/s to the east and north
along the edge of the IRAC shell. We identify two YSOs in early evolutionary
stages (Stage 0/I) within the densest molecular condensation. From the CO
kinematics, we find that the HII region is currently expanding with a velocity
of 3.4 km/s, implying that the shell's expansion age is ~1 Myr. This timescale
is in good agreement with numerical simulations of the expansion of the HII
region (Hosokawa et al. 2006). We conclude that the star formation on the
border of the shell is triggered by the expansion of the HII region.Comment: 9 pages, 10 figures, accepted for publication in Ap
Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge
The Magellanic Bridge is the nearest low-metallicity, tidally stripped
environment, offering a unique high-resolution view of physical conditions in
merging and forming galaxies. In this paper we present analysis of candidate
massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star
formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary
optical and near-IR photometry. While we definitely find YSOs in the Bridge,
the most massive are , found in the Large
Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears
decreasing, as the most massive YSOs are less massive than those formed in the
past. To investigate environmental effects on MSF, we have compared properties
of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge
are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical
counterparts, compared to only 56% of LMC sources with the same range of mass,
circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes
are evidently more porous or clumpy in the Bridge's low-metallicity
environment. Second, we have used whole samples of YSOs in the LMC and the
Bridge to estimate the probability of finding YSOs at a given \hi\ column
density, N(HI). We found that the LMC has higher probability than
the Bridge for N(HI) cm, but the trend reverses at
lower N(HI). Investigating whether this lower efficiency relative to HI is due
to less efficient molecular cloud formation, or less efficient cloud collapse,
or both, will require sensitive molecular gas observations.Comment: 41 pages, 20 figures, 6 tables; accepted for publication in ApJ;
several figures are in low resolution due to the size limit here and a high
resolution version can be downloaded via
http://www.astro.virginia.edu/~cc5ye/ms_bridge20140215.pd
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