507 research outputs found
Disk evolution in the Ori OB1 association
We analyze multi-band photometry of a subsample of low mass stars in the
associations Ori OB1a and 1b discovered during the CIDA Orion Variability
Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained
UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26
Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b.
We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and
18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected
at 10um while only one was detected at 18um. We estimate mass accretion rates
from the excess luminosity at U, and find that they are consistent with
determinations for a number of other associations, with or without high mass
star formation. The observed decrease of mass accretion rate with age is
qualitatively consistent with predictions of viscous evolution of accretion
disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to
Ori OB1a. This decrease implies that significant grain growth and settling
towards the midplane has taken place in the inner disks of Ori OB1. We compare
the SED of the star detected at both 10um and 18um with disk models for similar
stellar and accretion parameters. We find that the low <= 18 um fluxes of this
Ori OB1b star cannot be due to the smaller disk radius expected from viscous
evolution in the presence of the FUV radiation fields from the OB stars in the
association. Instead, we find that the disk of this star is essentially a flat
disk, with little if any flaring, indicating a a significant degree of dust
settling towards the midplane, as expected from dust evolution in
protoplanetary disks.Comment: 35 pages, 11 figures, to appear in the Astronomical Journal. Full
resolution figures in http://www.cida.ve/~briceno/publications
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
Signal-regulatory protein alpha is an anti-viral entry factor targeting viruses using endocytic pathways
Signal-regulatory protein alpha (SIRPA) is a well-known inhibitor of phagocytosis when it complexes with CD47 expressed on target cells. Here we show that SIRPA decreased in vitro infection by a number of pathogenic viruses, including New World and Old World arena-viruses, Zika virus, vesicular stomatitis virus and pseudoviruses bearing the Machupo virus, Ebola virus and SARS-CoV-2 glycoproteins, but not HSV-1, MLV or mNoV. Moreover, mice with targeted mutation of the Sirpa gene that renders it non-functional were more suscepti
ble to infection with the New World arenaviruses Junı´n virus vaccine strain Candid 1 and Tacaribe virus, but not MLV or mNoV. All SIRPA-inhibited viruses have in common the requirement for trafficking to a low pH endosomal compartment. This was clearly demonstrated with SARS-CoV-2 pseudovirus, which was only inhibited by SIRPA in cells in which it required trafficking to the endosome. Similar to its role in phagocytosis inhibition, SIRPA
decreased virus internalization but not binding to cell surface receptors. We also found that increasing SIRPA levels via treatment with IL-4 led to even greater anti-viral activity. These data suggest that enhancing SIRPA’s activity could be a target for anti-viral therapies
Higher viral load and infectivity increase risk of aerosol transmission for Delta and Omicron variants of SARS-CoV-2.
Airborne transmission of SARS-CoV-2 is an important route of infection. For the wildtype (WT) only a small proportion of those infected emitted large quantities of the virus. The currently prevalent variants of concern, Delta (B1.617.2) and Omicron (B.1.1.529), are characterized by higher viral loads and a lower minimal infective dose compared to the WT. We aimed to describe the resulting distribution of airborne viral emissions and to reassess the risk estimates for public settings given the higher viral load and infectivity.
We reran the Monte Carlo modelling to estimate viral emissions in the fine aerosol size range using available viral load data. We also updated our tool to simulate indoor airborne transmission of SARS-CoV-2 by including a CO2 calculator and recirculating air cleaning devices. We also assessed the consequences of the lower critical dose on the infection risk in public settings with different protection strategies.
Our modelling suggests that a much larger proportion of individuals infected with the new variants are high, very high or super-emitters of airborne viruses: for the WT, one in 1,000 infected was a super-emitter; for Delta one in 30; and for Omicron one in 20 or one in 10, depending on the viral load estimate used. Testing of the effectiveness of protective strategies in view of the lower critical dose suggests that surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting.
From an aerosol transmission perspective, the shift towards a larger proportion of very high emitting individuals, together with the strongly reduced critical dose, seem to be two important drivers of the aerosol risk, and are likely contributing to the observed rapid spread of the Delta and Omicron variants of concern. Reducing contacts, always wearing well-fitted FFP2 respirators when indoors, using ventilation and other methods to reduce airborne virus concentrations, and avoiding situations with loud voices seem critical to limiting these latest waves of the COVID-19 pandemic
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
Spectroscopy of Brown Dwarf Candidates in the rho Ophiuchi Molecular Core
We present an analysis of low resolution infrared spectra for 20 brown dwarf
candidates in the core of the Ophiuchi molecular cloud. Fifteen of the
sources display absorption-line spectra characteristic of late-type stars. By
comparing the depths of water vapor absorption bands in our candidate objects
with a grid of M dwarf standards, we derive spectral types which are
independent of reddening. Optical spectroscopy of one brown dwarf candidate
confirms the spectral type derived from the water bands. Combining their
spectral types with published near-infrared photometry, effective temperatures
and bolometric stellar luminosities are derived enabling us to place our sample
on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf
candidates in this diagram with two sets of theoretical models in order to
estimate their masses and ages. Considering uncertainties in placing the
candidates in the H-R diagram, six objects consistently lie in the brown dwarf
regime and another five objects lie in the transition region between stellar
and substellar objects. The ages inferred for the sample are consistent with
those derived for higher mass association members. Three of the newly
identified brown dwarfs display infrared excesses at =2.2 m
suggesting that young brown dwarfs can have active accretion disks. Comparing
our mass estimates of the brown dwarf candidates with those derived from
photometric data alone suggests that spectroscopy is an essential component of
investigations of the mass functions of young clusters.Comment: Astronomical Journal, in press: 25 pages, latex, 5 tables and 6
figures (separate
The dynamical evolution of fractal star clusters: the survival of substructure
We simulate the dynamics of fractal star clusters, in order to investigate
the evolution of substructure in recently formed clusters. The velocity
dispersion is found to be the key parameter determining the survival of
substructure. In clusters with a low initial velocity dispersion, the ensuing
collapse of the cluster tends to erase substructure, although some substructure
may persist beyond the collapse phase. In clusters with virial ratios of 0.5 or
higher, initial density substructure survives for several crossing times, in
virtually all cases. Even an initially homogeneous cluster can develop
substructure, if it is born with coherent velocity dispersion. These results
suggest that the simple initial conditions used for many sophisticated -body
simulations could be missing a very important and dramatic phase of star
cluster evolution.Comment: 12 pages, 9 figures, A&A in pres
Issues and Opportunities in Exotic Hadrons
The last few years have been witness to a proliferation of new results
concerning heavy exotic hadrons. Experimentally, many new signals have been
discovered that could be pointing towards the existence of tetraquarks,
pentaquarks, and other exotic configurations of quarks and gluons.
Theoretically, advances in lattice field theory techniques place us at the cusp
of understanding complex coupled-channel phenomena, modelling grows more
sophisticated, and effective field theories are being applied to an ever
greater range of situations. It is thus an opportune time to evaluate the
status of the field. In the following, a series of high priority experimental
and theoretical issues concerning heavy exotic hadrons is presented.Comment: White paper from INT workshop, "Modern Exotic Hadrons". References
added. Version to appear in Chinese Physics
Spitzer observations of the Orion OB1 association: second generation dust disks at 5-10 Myr
We report new Spitzer observations of intermediate mass stars in two regions
of the Orion OB1 association located in the subassociations OB1a (10 Myr)
and OB1b (5 Myr). In a representative sample of stars earlier than F5 of
both stellar groups, we find a population of stars surrounded of debris disks,
without excess in the IRAC bands and without emission lines in their optical
spectra, but with a varying degree of 24{\micron} excess. Comparing our samples
with 24{\micron} observations of intermediate mass stars in other stellar
groups, spanning a range of ages from 2.5 Myr to 150 Myr, we find that debris
disks are more frequent and have larger 24{\micron} excess at 10 Myr (OB1a).
This trend agrees with predictions of models of evolution of solids in the
outer regions of disks (30 AU), where large icy objects (1000 Km)
begin to form at 10 Myr; the presence of these objects in the disk
initiates a collisional cascade, producing enough dust particles to explain the
relatively large 24 {\micron} excess observed in OB1a. The dust luminosity
observed in the stellar groups older than 10 Myr declines roughly as predicted
by collisional cascade models. Combining Spitzer observations, optical spectra
and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star
(HD36444) with a large 24 {\micron} excess, both in OB1b. This last object
could be explained as a intermediate stage between HAeBe and true debris
systems or as a massive debris disk produced by a collision between two large
objects (1000 Km).Comment: 25 pages, 8 figures, 2 tables. To be published in Astrophysical
Journal (Acepted: 24 Jul 2006
The Mass Function of Newly Formed Stars (Review)
The topic of the stellar "original mass function" has a nearly 50 year
history,dating to the publication in 1955 of Salpeter's seminal paper. In this
review I discuss the many more recent results that have emerged on the initial
mass function (IMF), as it is now called, from studies over the last decade of
resolved populations in star forming regions and young open clusters.Comment: 9 pages, 1 figure; to appear in "The Dense Instellar Medium in
Galaxies -- 4'th Cologne-Bonn-Zermatt-Symposium" editted by S. Pfalzner, C.
Kramer, C. Straubmeier and A. Heithausen, Springer-Verlag (2004
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