1,163 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
Characterizing star formation activity in infrared dark cloud MSXDC G048.65-00.29
Infrared Dark Clouds (IRDCs), condensed regions of the ISM with high column
densities, low temperatures and high masses, are suspected sites of star
formation. Thousands of IRDCs have already been identified. To date, it has not
been resolved whether IRDCs always show star formation activity and, if so, if
massive star formation (> 8 solar masses) is the rule or the exception in
IRDCs. Previous analysis of sub-millimeter cores in the cloud MSXDC
G048.65-00.29 (G48.65) indicates embedded star formation activity. To
characterize this activity in detail, mid-infrared photometry (3-30 micron) has
been obtained with the Spitzer Space Telescope. This paper analyzes the point
sources seen in the 24 micron band, combined with counterparts or upper limits
at shorter and longer wavelengths. Data points in wavelength bands ranging from
1 up to 850 micron are used to compare each 24 micron source to a set of
Spectral Energy Distributions of Young Stellar Object (YSO) models. By
assessing the models that fit the data, an attempt is made to identify YSOs as
such and determine their evolutionary stages and stellar masses. A total of 17
sources are investigated, 13 of which are classified as YSOs, primarily - but
not exclusively - in an early embedded phase of star formation. The modeled
masses of the central stellar objects range from sub-solar to ~8 solar masses.
Every YSO is at less than 1 pc projected distance from its nearest YSO
neighbor. We conclude that IRDC G48.65 is a region of active star formation. We
find YSOs in various evolutionary phases, indicating that the star formation in
this cloud is not an instantaneous process. The inferred masses of the central
objects suggest that this IRDC hosts only low to intermediate mass YSOs and
none with masses exceeding ~8 solar masses.Comment: 10 pages, 6 figures; v2: minor editorial changes to match published
versio
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
Sky maps without anisotropies in the cosmic microwave background are a better fit to WMAP's uncalibrated time ordered data than the official sky maps
The purpose of this reanalysis of the WMAP uncalibrated time ordered data
(TOD) was two fold. The first was to reassess the reliability of the detection
of the anisotropies in the official WMAP sky maps of the cosmic microwave
background (CMB). The second was to assess the performance of a proposed
criterion in avoiding systematic error in detecting a signal of interest. The
criterion was implemented by testing the null hypothesis that the uncalibrated
TOD was consistent with no anisotropies when WMAP's hourly calibration
parameters were allowed to vary. It was shown independently for all 20 WMAP
channels that sky maps with no anisotropies were a better fit to the TOD than
those from the official analysis. The recently launched Planck satellite should
help sort out this perplexing result.Comment: 11 pages with 1 figure and 2 tables. Extensively rewritten to explain
the research bette
Triggered star formation and Young Stellar Population in Bright-Rimmed Cloud SFO 38
We have investigated the young stellar population in and around SFO 38, one
of the massive globules located in the northern part of the Galactic HII region
IC 1396, using the Spitzer IRAC and MIPS observations (3.6 to 24 micron) and
followed up with ground based optical photometric and spectroscopic
observations. Based on the IRAC and MIPS colors and H-alpha emission we
identify ~45 Young Stellar Objects (Classes 0/I/II) and 13 probable Pre Main
Sequence candidates. We derive the spectral types (mostly K- and M-type stars),
effective temperatures and individual extinction of the relatively bright and
optically visible Class II objects. Based on optical photometry and theoretical
isochrones, we estimate the spread in stellar ages to be between 1--8 Myr with
a median age of 3 Myr and a mass distribution of 0.3--2.2 Msun with a median
value around 0.5 Msun. Using the width of the H-alpha emission line measured at
10% peak intensity, we derive the mass accretion rates of individual objects to
be between 10^{-10} to 10^{-8} Msun/yr. From the continuum-subtracted H-alpha
line image, we find that the H-alpha emission of the globule is not spatially
symmetric with respect to the O type ionizing star HD 206267. We clearly detect
an enhanced concentration of YSOs closer to the southern rim of SFO~38 and
identify an evolutionary sequence of YSOs from the rim to the dense core of the
cloud, with most of the Class II objects located at the bright rim. The YSOs
appear to be aligned along two different directions towards the O6.5V type star
HD 206267 and the B0V type star HD 206773. This is consistent with the
Radiation Driven Implosion (RDI) model for triggered star formation. (Abridged)Comment: Accepted for publication in Ap
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 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
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
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
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
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