585 research outputs found
Pulsed Accretion in the T Tauri Binary TWA 3A
TWA 3A is the most recent addition to a small group of young binary systems
that both actively accrete from a circumbinary disk and have spectroscopic
orbital solutions. As such, it provides a unique opportunity to test binary
accretion theory in a well-constrained setting. To examine TWA 3A's
time-variable accretion behavior, we have conducted a two-year, optical
photometric monitoring campaign, obtaining dense orbital phase coverage (~20
observations per orbit) for ~15 orbital periods. From U-band measurements we
derive the time-dependent binary mass accretion rate, finding bursts of
accretion near each periastron passage. On average, these enhanced accretion
events evolve over orbital phases 0.85 to 1.05, reaching their peak at
periastron. The specific accretion rate increases above the quiescent value by
a factor of ~4 on average but the peak can be as high as an order of magnitude
in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in
good agreement with numerical simulations of binary accretion with similar
orbital parameters. In these simulations, periastron accretion bursts are
fueled by periodic streams of material from the circumbinary disk that are
driven by the binary orbit. We find that TWA 3A's average accretion behavior is
remarkably similar to DQ Tau, another T Tauri binary with similar orbital
parameters, but with significantly less variability from orbit to orbit. This
is only the second clear case of orbital-phase-dependent accretion in a T Tauri
binary.Comment: 6 pages, 4 figure
Time Reversal Invariance Violation in Neutron Deuteron Scattering
Time reversal invariance violating (TRIV) effects for low energy elastic
neutron deuteron scattering are calculated for meson exchange and EFT-type of
TRIV potentials in a Distorted Wave Born Approximation, using realistic
hadronic strong interaction wave functions, obtained by solving three-body
Faddeev equations in configuration space. The relation between TRIV and parity
violating observables are discussed
Constraints from Dust Mass and Mass Accretion Rate Measurements on Angular Momentum Transport in Protoplanetary Disks
We investigate the relation between disk mass and mass accretion rate to
constrain the mechanism of angular momentum transport in protoplanetary disks.
Dust mass and mass accretion rate in Chamaeleon I are correlated with a slope
close to linear, similar to the one recently identified in Lupus. We
investigate the effect of stellar mass and find that the intrinsic scatter
around the best-fit Mdust-Mstar and Macc-Mstar relations is uncorrelated. Disks
with a constant alpha viscosity can fit the observed relations between dust
mass, mass accretion rate, and stellar mass, but over-predict the strength of
the correlation between disk mass and mass accretion rate when using standard
initial conditions. We find two possible solutions. 1) The observed scatter in
Mdust and Macc is not primoridal, but arises from additional physical processes
or uncertainties in estimating the disk gas mass. Most likely grain growth and
radial drift affect the observable dust mass, while variability on large time
scales affects the mass accretion rates. 2) The observed scatter is primordial,
but disks have not evolved substantially at the age of Lupus and Chamaeleon I
due to a low viscosity or a large initial disk radius. More accurate estimates
of the disk mass and gas disk sizes in a large sample of protoplanetary disks,
either through direct observations of the gas or spatially resolved
multi-wavelength observations of the dust with ALMA, are needed to discriminate
between both scenarios or to constrain alternative angular momentum transport
mechanisms such as MHD disk winds.Comment: See also the paper by Lodato et a
X-Shooter study of accretion in Chamaeleon I
DF acknowledges support from the Italian Ministry of Science and Education (MIUR), project SIR (RBSI14ZRHR) and from the ESTEC Faculty Visiting Scientist Programme.We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star-forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to ~700 nm. The dependence of accretion on stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than inmany studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter between Chamaeleon I and Lupus.Complete samples in Chamaeleon I and Lupus are needed to determine whether the difference in scatter of accretion rates and the lack of evolutionary trends are not influenced by sample selection.PostprintPeer reviewe
Warm gas towards young stellar objects in Corona Australis - Herschel/PACS observations from the DIGIT key programme
The effects of external irradiation on the chemistry and physics in the
protostellar envelope around low-mass young stellar objects are poorly
understood. The Corona Australis star-forming region contains the R CrA dark
cloud, comprising several low-mass protostellar cores irradiated by an
intermediate-mass young star. We study the effects on the warm gas and dust in
a group of low-mass young stellar objects from the irradiation by the young
luminous Herbig Be star R CrA. Herschel/PACS far-infrared datacubes of two
low-mass star-forming regions in the R CrA dark cloud are presented. The
distribution of CO, OH, H2O, [C II], [O I], and continuum emission is
investigated. We have developed a deconvolution algorithm which we use to
deconvolve the maps, separating the point-source emission from the extended
emission. We also construct rotational diagrams of the molecular species. By
deconvolution of the Herschel data, we find large-scale (several thousand AU)
dust continuum and spectral line emission not associated with the point
sources. Similar rotational temperatures are found for the warm CO (
K), hot CO ( K), OH ( K), and H2O ( K) emission,
respectively, in the point sources and the extended emission. The rotational
temperatures are also similar to what is found in other more isolated cores.
The extended dust continuum emission is found in two ridges similar in extent
and temperature to molecular mm emission, indicative of external heating from
the Herbig Be star R CrA. Our results show that a nearby luminous star does not
increase the molecular excitation temperatures in the warm gas around a young
stellar object (YSO). However, the emission from photodissociation products of
H2O, such as OH and O, is enhanced in the warm gas associated with these
protostars and their surroundings compared to similar objects not suffering
from external irradiation.Comment: 37 pages, accepted for publication in A&
The CP-violating asymmetry in \eta\to\pi^+ \pi^- e^+e^-
We study the CP-violating asymmetry {\cal A}_{\rm CP}, which arises, in
\eta\to\pi^+\pi^- e^+e^-, from the angular correlation of the e^+ e^- and
\pi^+\pi^- planes due to the interference between the magnetic and electric
decay amplitudes. With the phenomenologically determined magnetic amplitude and
branching ratio as input, the asymmetry, induced by the electric bremsstrahlung
amplitude through the CP-violating decay \eta\to\pi^+\pi^-, and by an
unconventional tensor type operator, has been estimated respectively. The upper
bound of {\cal A}_{\rm CP} from the former is about 10^{-3}, and the asymmetry
from the latter might be up to O(10^{-2}). One can therefore expect that this
CP asymmetry would be an interesting CP-violating observable for the future
precise measurements in the \eta factories.Comment: LaTeX, 6 pages. One reference corrected, and some new references
adde
An ALMA Survey of faint disks in the Chamaeleon I star-forming region: Why are some Class II disks so faint?
ALMA surveys of nearby star-forming regions have shown that the dust mass in
the disk is correlated with the stellar mass, but with a large scatter. This
scatter could indicate either different evolutionary paths of disks or
different initial conditions within a single cluster. We present ALMA Cycle 3
follow-up observations for 14 Class II disks that were low S/N detections or
non-detections in our Cycle 2 survey of the Myr-old Chamaeleon I
star-forming region. With 5 times better sensitivity, we detect millimeter dust
continuum emission from six more sources and increase the detection rate to
94\% (51/54) for Chamaeleon I disks around stars earlier than M3. The
stellar-disk mass scaling relation reported in \citet{pascucci2016} is
confirmed with these updated measurements. Faint outliers in the
-- plane include three non-detections (CHXR71, CHXR30A, and T54)
with dust mass upper limits of 0.2 M and three very faint disks
(CHXR20, ISO91, and T51) with dust masses M. By
investigating the SED morphology, accretion property and stellar multiplicity,
we suggest for the three millimeter non-detections that tidal interaction by a
close companion (100 AU) and internal photoevaporation may play a role in
hastening the overall disk evolution. The presence of a disk around only the
secondary star in a binary system may explain the observed stellar SEDs and low
disk masses for some systems.Comment: ApJ accepte
UV excess measures of accretion onto young very low-mass stars and brown dwarfs
Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown
dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission
arising in the Balmer and Paschen continua yields mass accretion rates ranging
from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS
spectra of roughly solar-mass accretors with accretion rates that range from
2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A
leads to a higher contrast between the accretion and photospheric emission
relative to higher-mass counterparts. The mass accretion rates measured here
are systematically 4-7 times larger than those from H-alpha emission line
profiles, with a difference that is consistent with but unlikely to be
explained by the uncertainty in both methods. The accretion luminosity
correlates well with many line luminosities, including high Balmer and many He
I lines. Correlations of the accretion rate with H-alpha 10% width and line
fluxes show a large amount of scatter. Our results and previous accretion rate
measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for
accretors in the Taurus Molecular Cloud.Comment: 13 pages text, 15 tables, 14 figures. Accepted by Ap
Differential effects of caffeine on hair shaft elongation, matrix and outer root sheath keratinocyte proliferation, and TGF-beta2-/IGF-1-mediated regulation of hair cycle in male and female human hair follicles in vitro.
BACKGROUND: Caffeine reportedly counteracts the suppression of hair shaft production by testosterone in organ-cultured male human hair follicles (HFs). OBJECTIVES: We aimed at investigating the impact of caffeine a) on additional key hair growth parameters, b) on major hair growth-regulatory growth factors and c) on male versus female HFs in the presence of testosterone. METHODS: Microdissected male and female human scalp HFs were treated in serum-free organ culture for 120 h with testosterone alone (0,5 mug/ml) or in combination with caffeine (0.005-0.0005%), and effects on hair shaft elongation, HF cycling (i.e. anagen-catagen transition), hair matrix keratinocyte proliferation and expression of a key catagen inducer, transforming growth factor beta2 (TGF-beta2), and anagen-prolonging insulin-like growth factor 1 (IGF-1) were evaluated by quantitative (immuno-) histomorphometry. Caffeine effects were further investigated in human outer root sheath keratinocytes (ORSK). RESULTS: Caffeine enhanced hair shaft elongation, prolonged anagen duration and stimulated hair matrix keratinocyte proliferation. Female HFs showed higher sensitivity to caffeine compared to male HFs. Caffeine counteracted testosterone-enhanced TGF-beta2 protein expression in male HFs. In female HFs, testosterone failed to induce TGF-beta2 expression, while caffeine reduced it. In male and female HFs, caffeine enhanced IGF-1 protein expression. In ORSK, caffeine stimulated cell proliferation, inhibited apoptosis/necrosis, up-regulated IGF-1 gene expression and protein secretion, while TGF-beta2 protein secretion was down-regulated. CONCLUSIONS: This study reveals new growth-promoting effects of caffeine on human hair follicles of both genders at different (molecular, cellular and organ) levels. This article is protected by copyright. All rights reserved
Constraints on the Universal Contact Interaction
Forces beyond those of the standard model may manifest themselves at low
energies as four-fermion contact interactions. If these new forces are
independent of colour and flavour quantum numbers including baryon and lepton
number, then all low energy constraints, arising from quark-lepton
universality, flavour-changing neutral currents and atomic parity violation are
evaded. This is due to the global U(45) symmetry which the standard model
exhibits in the limit of vanishing gauge and Yukawa couplings. The
corresponding contact interaction is a unique current-current interaction.
Constraints from LEP2 imply that this universal contact interaction cannot be
the origin of the recently observed high- events at HERA.Comment: 6 pages Latex, no figure
- …