772 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
Turbulence and Accretion: a High-resolution Study of the B5 Filaments
High-resolution observations of the Perseus B5 "core" have previously
revealed that this subsonic region actually consists of several filaments that
are likely in the process of forming a quadruple stellar system. Since subsonic
filaments are thought to be produced at the pc sonic scale by
turbulent compression, a detailed kinematic study is crucial to test such a
scenario in the context of core and star formation. Here we present a detailed
kinematic follow-up study of the B5 filaments at a 0.009 pc resolution using
the VLA and GBT combined observations fitted with multi-component spectral
models. Using precisely identified filament spines, we find a remarkable
resemblance between the averaged width profiles of each filament and
Plummer-like functions, with filaments possessing FWHM widths of
pc. The velocity dispersion profiles of the filaments also show decreasing
trends towards the filament spines. Moreover, the velocity gradient field in B5
appears to be locally well ordered ( pc) but globally complex, with
kinematic behaviors suggestive of inhomogeneous turbulent accretion onto
filaments and longitudinal flows towards a local overdensity along one of the
filaments.Comment: 27 pages, 12 figures, published in The Astrophysical Journa
Cluster J Mycobacteriophages: Intron Splicing in Capsid and Tail Genes
Bacteriophages isolated on Mycobacterium smegmatis mc2155 represent many distinct genomes sharing little or no DNA sequence similarity. The genomes are architecturally mosaic and are replete with genes of unknown function. A new group of genomes sharing substantial nucleotide sequences constitute Cluster J. The six mycobacteriophages forming Cluster J are morphologically members of the Siphoviridae, but have unusually long genomes ranging from 106.3 to 117 kbp. Reconstruction of the capsid by cryo-electron microscopy of mycobacteriophage BAKA reveals an icosahedral structure with a triangulation number of 13. All six phages are temperate and homoimmune, and prophage establishment involves integration into a tRNA-Leu gene not previously identified as a mycobacterial attB site for phage integration. The Cluster J genomes provide two examples of intron splicing within the virion structural genes, one in a major capsid subunit gene, and one in a tail gene. These genomes also contain numerous free-standing HNH homing endonuclease, and comparative analysis reveals how these could contribute to genome mosaicism. The unusual Cluster J genomes provide new insights into phage genome architecture, gene function, capsid structure, gene mobility, intron splicing, and evolution. © 2013 Pope et al
The High-Order-Multiplicity of Unusually Wide M-dwarf Binaries: Eleven New Triple and Quadruple Systems
M-dwarfs in extremely wide binary systems are very rare, and may thus have
different formation processes from those found as single stars or close
binaries in the field. In this paper we search for close companions to a new
sample of 36 extremely wide M-dwarf binaries, covering a spectral type range of
M1 to M5 and a separation range of 600 - 6500 AU. We discover 10 new triple
systems and one new quadruple system. We carefully account for selection
effects including proper motion, magnitude limits, the detection of close
binaries in the SDSS, and other sample biases. The bias-corrected total
high-order-multiple fraction is 45% (+18%/-16%) and the bias-corrected
incidence of quadruple systems is < 5%, both statistically compatible with that
found for the more common close M-dwarf multiple systems. Almost all the
detected companions have similar masses to their primaries, although two very
low mass companions, including a candidate brown dwarf, are found at relatively
large separations. We find that the close-binary separation distribution is
strongly peaked towards < 30AU separations. There is marginally significant
evidence for a change in high-order M-dwarf multiplicity with binding energy
and total mass. We also find 2-sigma evidence of an unexpected increased
high-order-multiple fraction for the widest targets in our survey, with a
high-order-multiple fraction of 21% (+17%/-7%) for systems with separations up
to 2000AU, compared to 77% (+9%/-22%) for systems with separations > 4000AU.
These results suggest that the very widest M-dwarf binary systems need higher
masses to form or to survive.Comment: 11 pages, 14 figures, accepted for publication in Ap
The Green Bank Ammonia Survey: Unveiling the Dynamics of the Barnard 59 star-forming Clump
Understanding the early stages of star formation is a research field of
ongoing development, both theoretically and observationally. In this context,
molecular data have been continuously providing observational constraints on
the gas dynamics at different excitation conditions and depths in the sources.
We have investigated the Barnard 59 core, the only active site of star
formation in the Pipe Nebula, to achieve a comprehensive view of the kinematic
properties of the source. These information were derived by simultaneously
fitting ammonia inversion transition lines (1,1) and (2,2). Our analysis
unveils the imprint of protostellar feedback, such as increasing line widths,
temperature and turbulent motions in our molecular data. Combined with
complementary observations of dust thermal emission, we estimate that the core
is gravitationally bound following a virial analysis. If the core is not
contracting, another source of internal pressure, most likely the magnetic
field, is supporting it against gravitational collapse and limits its star
formation efficiency.Comment: 18 pages, 18 figure
RTL551 Treatment of EAE Reduces CD226 and T-bet+ CD4 T Cells in Periphery and Prevents Infiltration of T-bet+ IL-17, IFN-γ Producing T Cells into CNS
Recombinant T cell receptor ligands (RTLs) that target encephalitogenic T-cells can reverse clinical and histological signs of EAE, and are currently in clinical trials for treatment of multiple sclerosis. To evaluate possible regulatory mechanisms, we tested effects of RTL therapy on expression of pathogenic and effector T-cell maturation markers, CD226, T-bet and CD44, by CD4+ Th1 cells early after treatment of MOG-35-55 peptide-induced EAE in C57BL/6 mice. We showed that 1–5 daily injections of RTL551 (two-domain I-Ab covalently linked to MOG-35-55 peptide), but not the control RTL550 (“empty” two-domain I-Ab without a bound peptide) or Vehicle, reduced clinical signs of EAE, prevented trafficking of cells outside the spleen, significantly reduced the frequency of CD226 and T-bet expressing CD4+ T-cells in blood and inhibited expansion of CD44 expressing CD4+ T-cells in blood and spleen. Concomitantly, RTL551 selectively reduced CNS inflammatory lesions, absolute numbers of CNS infiltrating T-bet expressing CD4+ T-cells and IL-17 and IFN-γ secretion by CNS derived MOG-35-55 reactive cells cultured ex vivo. These novel results demonstrate that a major effect of RTL therapy is to attenuate Th1 specific changes in CD4+ T-cells during EAE and prevent expansion of effector T-cells that mediate clinical signs and CNS inflammation in EAE
Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37
If star formation proceeds by thermal fragmentation and the subsequent
gravitational collapse of the individual fragments, how is it possible to form
fragments massive enough for O and B stars in a typical star-forming molecular
cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3)
and temperatures (10K)? We test the hypothesis that a first generation of
low-mass stars may heat the gas enough that subsequent thermal fragmentation
results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and
SMA observations of the massive star-forming region G8.68-0.37 with radiative
transfer modeling to derive the present-day conditions in the region and use
this to infer the conditions in the past, at the time of core formation.
Assuming the current mass/separation of the observed cores equals the
fragmentation Jeans mass/length and the region's average density has not
changed, requires the gas temperature to have been 100K at the time of
fragmentation. The postulated first-generation of low-mass stars would still be
around today, but the number required to heat the cloud exceeds the limits
imposed by the observations. Several lines of evidence suggest the observed
cores in the region should eventually form O stars yet none have sufficient raw
material. Even if feedback may have suppressed fragmentation, it was not
sufficient to halt it to this extent. To develop into O stars, the cores must
obtain additional mass from outside their observationally defined boundaries.
The observations suggest they are currently fed via infall from the very
massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the
star-forming cores. This suggests that massive stars do not form in the
collapse of individual massive fragments, but rather in smaller fragments that
themselves continue to gain mass by accretion from larger scales.Comment: 23 pages, 14 figures. Accepted for publication in Ap
The Age, Stellar Content and Star Formation Timescale of the B59 Dense Core
We have used moderate resolution, near-infrared spectra from the SpeX
spectrograph on the NASA Infrared Telescope facility to characterize the
stellar content of Barnard 59 (B59), the most active star-forming core in the
Pipe Nebula. Measuring luminosity and temperature sensitive features in the
spectra of 20 candidate YSOs, we identified likely background giant stars and
measured each star's spectral type, extinction, and NIR continuum excess. We
find that B59 is composed of late type (K4-M6) low-mass (0.9--0.1 M_sun) YSOs
whose median stellar age is comparable to, if not slightly older than, that of
YSOs within the Rho Oph, Taurus, and Chameleon star forming regions. Deriving
absolute age estimates from pre-main sequence models computed by D'Antona et
al., and accounting only for statistical uncertainties, we measure B59's median
stellar age to be 2.6+/-0.8 Myrs. Including potential systematic effects
increases the error budget for B59's median (DM98) stellar age to 2.6+4.1/-2.6
Myrs. We also find that the relative age orderings implied by pre-main sequence
evolutionary tracks depend on the range of stellar masses sampled, as model
isochrones possess significantly different mass dependencies. The maximum
likelihood median stellar age we measure for B59, and the region's observed gas
properties, suggest that the B59 dense core has been stable against global
collapse for roughly 6 dynamical timescales, and is actively forming stars with
a star formation efficiency per dynamical time of ~6%. This maximum likelihood
value agrees well with recent star formation simulations that incorporate
various forms of support against collapse, such as sub-critical magnetic
fields, outflows, and radiative feedback from protostellar heating. [abridged]Comment: 20 pages, 12 figures, accepted for publication in the Astrophysical
Journal; updated to amend acknowledgment
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