6,796 research outputs found
Protostellar accretion traced with chemistry. High resolution C18O and continuum observations towards deeply embedded protostars in Perseus
Context: Understanding how accretion proceeds is a key question of star
formation, with important implications for both the physical and chemical
evolution of young stellar objects. In particular, very little is known about
the accretion variability in the earliest stages of star formation.
Aims: To characterise protostellar accretion histories towards individual
sources by utilising sublimation and freeze-out chemistry of CO.
Methods: A sample of 24 embedded protostars are observed with the
Submillimeter Array (SMA) in context of the large program "Mass Assembly of
Stellar Systems and their Evolution with the SMA" (MASSES). The size of the
CO emitting region, where CO has sublimated into the gas-phase, is
measured towards each source and compared to the expected size of the region
given the current luminosity. The SMA observations also include 1.3 mm
continuum data, which are used to investigate whether a link can be established
between accretion bursts and massive circumstellar disks.
Results: Depending on the adopted sublimation temperature of the CO ice,
between 20% and 50% of the sources in the sample show extended CO
emission indicating that the gas was warm enough in the past that CO sublimated
and is currently in the process of refreezing; something which we attribute to
a recent accretion burst. Given the fraction of sources with extended CO
emission, we estimate an average interval between bursts of 20000-50000 yr,
which is consistent with previous estimates. No clear link can be established
between the presence of circumstellar disks and accretion bursts, however the
three closest known binaries in the sample (projected separations <20 AU) all
show evidence of a past accretion burst, indicating that close binary
interactions may also play a role in inducing accretion variability.Comment: Accepted for publication in A&A, 21 pages, 13 figure
CARMA Large Area Star Formation Survey: Observational Analysis of Filaments in the Serpens South Molecular Cloud
We present the N2H+(J=1-0) map of the Serpens South molecular cloud obtained
as part of the CARMA Large Area Star Formation Survey (CLASSy). The
observations cover 250 square arcminutes and fully sample structures from 3000
AU to 3 pc with a velocity resolution of 0.16 km/s, and they can be used to
constrain the origin and evolution of molecular cloud filaments. The spatial
distribution of the N2H+ emission is characterized by long filaments that
resemble those observed in the dust continuum emission by Herschel. However,
the gas filaments are typically narrower such that, in some cases, two or three
quasi-parallel N2H+ filaments comprise a single observed dust continuum
filament. The difference between the dust and gas filament widths casts doubt
on Herschel ability to resolve the Serpens South filaments. Some molecular
filaments show velocity gradients along their major axis, and two are
characterized by a steep velocity gradient in the direction perpendicular to
the filament axis. The observed velocity gradient along one of these filaments
was previously postulated as evidence for mass infall toward the central
cluster, but these kind of gradients can be interpreted as projection of
large-scale turbulence.Comment: 12 pages, 4 figures, published in ApJL (July 2014
Increasing Campus Sense of Belonging through LLC Participation: To Gems Camp We Go
Gemstone seeks to develop students’ research and collaboration skills, foster leadership opportunities, and provide a sense of belonging on a large campus utilizing the Best Practices Model (BPM) and a variety of high impact practices (HIPs), like a Gemstone-specific orientation program called Gems Camp. While studies have demonstrated that LLCs increase students’ sense of belonging, the goal of this study is to explicitly test via propensity score matching if (a) enrollment in the Gemstone Honors Program increases sense of belonging compared to university students not in Gemstone and (b) attendance at Gems Camp increases sense of belonging in Gemstone students. Gemstone students (N=221) had an increased sense of belonging compared to matched university students (N=221). Moreover, Gemstone students who attended Gems Camp (N=92) had an increased sense of belonging compared to matched Gemstone students who did not attend Gems Camp (N=92). In conclusion, the Gemstone Honors Program is an example of an LLC with scaffolded high impact practices, such as intentional first year programming, undergraduate research, and collaborative projects, that promotes an increase in students’ sense of belonging, providing a model for other LLCs to consider in their programming efforts
Herschel/PACS Imaging of Protostars in the HH 1-2 Outflow Complex
We present 70 and 160 micron Herschel science demonstration images of a field
in the Orion A molecular cloud that contains the prototypical Herbig-Haro
objects HH 1 and 2, obtained with the Photodetector Array Camera and
Spectrometer (PACS). These observations demonstrate Herschel's unprecedented
ability to study the rich population of protostars in the Orion molecular
clouds at the wavelengths where they emit most of their luminosity. The four
protostars previously identified by Spitzer 3.6-40 micron imaging and
spectroscopy are detected in the 70 micron band, and three are clearly detected
at 160 microns. We measure photometry of the protostars in the PACS bands and
assemble their spectral energy distributions (SEDs) from 1 to 870 microns with
these data, Spitzer spectra and photometry, 2MASS data, and APEX sub-mm data.
The SEDs are fit to models generated with radiative transfer codes. From these
fits we can constrain the fundamental properties of the protostars. We find
luminosities in the range 12-84 L_sun and envelope densities spanning over two
orders of magnitude. This implies that the four protostars have a wide range of
envelope infall rates and evolutionary states: two have dense, infalling
envelopes, while the other two have only residual envelopes. We also show the
highly irregular and filamentary structure of the cold dust and gas surrounding
the protostars as traced at 160 microns.Comment: 6 pages, 4 figures, accepted for publication in the A&A Herschel
special issu
Financial Transaction Tax: Small is Beautiful
The case for taxing financial transactions merely to raise more revenues from the financial sector is not particularly strong. Better alternatives to tax the financial sector are likely to be available. However, a tax on financial transactions could be justified in order to limit socially
undesirable transactions when more direct means of doing so are unavailable for political or
practical reasons. Some financial transactions are indeed likely to do more harm than good,
especially when they contribute to the systemic risk of the financial system. However, such a
financial transaction tax should be very small, much smaller than the negative externalities in
question, because it is a blunt instrument that also drives out socially useful transactions.
There is a case for taxing over-the-counter derivative transactions at a somewhat higher rate
than exchange-based derivative transactions. More targeted remedies to drive out socially
undesirable transactions should be sought in parallel, which would allow, after their
implementation, to reduce or even phase out financialtransaction taxes
Topobiology of Human Pigmentation: P-Cadherin Selectively Stimulates Hair Follicle Melanogenesis
On the lease rate, convenience yield and speculative effects in the gold futures market
By examining data on the gold forward offered rate (GOFO) and lease rates over the period 1996- 2009, we conclude that the convenience yield of gold is better approximated by the lease rate than the interest-adjusted spread of Fama & French (1983). Using the latter quantity, we study the relationship between gold leasing and the level of COMEX discretionary inventory and exhibit that lease rates are negatively related to inventories. We also show that Futures prices have increasingly exceeded forward prices over the period, and this effect increases with the speculative pressure and the maturity of the contracts
Reverberation Mapping Measurements of Black Hole Masses in Six Local Seyfert Galaxies
We present the final results from a high sampling rate, multi-month,
spectrophotometric reverberation mapping campaign undertaken to obtain either
new or improved Hbeta reverberation lag measurements for several relatively
low-luminosity AGNs. We have reliably measured thetime delay between variations
in the continuum and Hbeta emission line in six local Seyfert 1 galaxies. These
measurements are used to calculate the mass of the supermassive black hole at
the center of each of these AGNs. We place our results in context to the most
current calibration of the broad-line region (BLR) R-L relationship, where our
results remove outliers and reduce the scatter at the low-luminosity end of
this relationship. We also present velocity-resolved Hbeta time delay
measurements for our complete sample, though the clearest velocity-resolved
kinematic signatures have already been published.Comment: 52 pages (AASTeX: 29 pages of text, 8 tables, 7 figures), accepted
for publication in the Astrophysical Journa
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