512 research outputs found
On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs
We investigate the detectability of Zeeman broadening in optical Stokes I
spectra of slowly rotating sun-like stars. To this end, we apply the LTE
spectral line inversion package SPINOR to very-high quality CES data and
explore how fit quality depends on the average magnetic field, Bf .
One-component (OC) and two-component (TC) models are adopted. In OC models, the
entire surface is assumed to be magnetic. Under this assumption, we determine
formal 3{\sigma} upper limits on the average magnetic field of 200 G for the
Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of ~
500 G is found for 59 Vir (G0V), and of ~ 1000 G for HD 68456 (F6V). A
distinction between magnetic and non-magnetic regions is made in TC models,
while assuming a homogeneous distribution of both components. In our TC
inversions of 59 Vir, we investigate three cases: both components have equal
temperatures; warm magnetic regions; cool magnetic regions. Our TC model with
equal temperatures does not yield significant improvement over OC inversions
for 59 Vir. The resulting Bf values are consistent for both. Fit quality is
significantly improved, however, by using two components of different
temperatures. The inversions for 59 Vir that assume different temperatures for
the two components yield results consistent with 0 - 450 G at the formal
3{\sigma} confidence level. We thus find a model dependence of our analysis and
demonstrate that the influence of an additional temperature component can
dominate over the Zeeman broadening signature, at least in optical data.
Previous comparable analyses that neglected effects due to multiple temperature
components may be prone to the same ambiguities.Comment: 18 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines
We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists
of spectral ranges 40 A wide centered on 1345, 1400, 1497, 1550, and 1900 A.
These UV spectra show strong SiIV, and CIV emission, and large quantities of
sharp (~40 km/s) H2 lines. All the H2 lines belong to the Lyman band and all
the observed lines are single peaked and optically thin. The averages of all
the H2 lines centroids for each star are negative which may indicate that they
come from an outflow. We interpret the emission in H2 as being due to
fluorescence, mostly by Ly_alpha, and identify seven excitation routes within 4
A of that line. We obtain column densities (10^12 to 10^15 cm^-2) and optical
depths (~1 or less) for each exciting transition. We conclude that the
populations are far from being in thermal equilibrium. We do not observe any
lines excited from the far blue wing of Ly_alpha, which implies that the
molecular features are excited by an absorbed profile. SiIV and CIV (corrected
for H2 emission) have widths of ~200 km/s, and an array of centroids
(blueshifted lines, centered, redshifted). These characteristics are difficult
to understand in the context of current models of the accretion shock. For DR
Tau we observe transient strong blueshifted emission, perhaps the a result of
reconnection events in the magnetosphere. We also see evidence of multiple
emission regions for the hot lines. While CIV is optically thin in most stars
in our sample, SiIV is not. However, CIV is a good predictor of SiIV and H2
emission. We conclude that most of the flux in the hot lines may be due to
accretion processes, but the line profiles can have multiple and variable
components.Comment: 67 pages, 19 figures, Accepted in Ap
The model of dynamo with small number of modes and magnetic activity of T Tauri stars
The model that describes operation of dynamo in fully convective stars is
presented. It is based on representation of stellar magnetic field as a
superposition of finite number of poloidal and toroidal free damping modes. In
the frame of adopted low of stellar differential rotation we estimated minimal
value of dynamo number D, starting from which generation of cyclic magnetic
field in stars without radiative core is possible. We also derived expression
for period of the cycle. It was found that dynamo cycles of fully convective
stars and stars with thin convective envelopes differ in a qualitative way: 1)
distribution of spots over latitude during the cycle is different in these
stars; 2) the model predicts that spot formation in fully convective stars
should be strongly suppressed at some phases of the cycle.
We have analyzed historical lightcurve of WTTS star V410 Tau and found that
long term activity of the star is not periodic process. Rather one can speak
about quasi cyclic activity with characteristic time of yr and chaotic
component over imposed. We concluded also that redistribution of cool spots
over longitude is the reason of long term variations of V410 Tau brightness. It
means that one can not compare directly results of photometric observations
with predictions of our axially symmetric (for simplicity) model which allows
to investigate time evolution of spot's distribution over latitude. We then
discuss what kind of observations and in which way could be used to check
predictions of the dynamo theory.Comment: 18 pages, 5 figures, accepted to Astron. Let
Electrodeposition of Silver Amalgam on Thin Gold Film Electrodes for Voltammetric Detection of 4Nitrophenol and DNA Labeled with Osmium TetroxideBipyridine Complex
Herein, electrodes made of vapordeposited thin gold films (vAuE) were used as an alternative substrate for the electrodeposition of silver amalgam particles (AgAPs), next to indium tin oxide and pyrolytic graphite, which are already used. The conditions and parameters of double pulse chronoamperometry were optimized for the mostsensitive voltammetric detection of 4nitrophenol (4NP). The resulting electrodes were characterized by scanning electron microscope with energy dispersive Xray spectroscopy
TRANSFAC(Âź) and its module TRANSCompel(Âź): transcriptional gene regulation in eukaryotes
The TRANSFAC(Âź) database on transcription factors, their binding sites, nucleotide distribution matrices and regulated genes as well as the complementing database TRANSCompel(Âź) on composite elements have been further enhanced on various levels. A new web interface with different search options and integrated versions of Matchâą and Patchâą provides increased functionality for TRANSFAC(Âź). The list of databases which are linked to the common GENE table of TRANSFAC(Âź) and TRANSCompel(Âź) has been extended by: Ensembl, UniGene, EntrezGene, HumanPSDâą and TRANSPROâą. Standard gene names from HGNC, MGI and RGD, are included for human, mouse and rat genes, respectively. With the help of InterProScan, Pfam, SMART and PROSITE domains are assigned automatically to the protein sequences of the transcription factors. TRANSCompel(Âź) contains now, in addition to the COMPEL table, a separate table for detailed information on the experimental EVIDENCE on which the composite elements are based. Finally, for TRANSFAC(Âź), in respect of data growth, in particular the gain of Drosophila transcription factor binding sites (by courtesy of the Drosophila DNase I footprint database) and of Arabidopsis factors (by courtesy of DATF, Database of Arabidopsis Transcription Factors) has to be stressed. The here described public releases, TRANSFAC(Âź) 7.0 and TRANSCompel(Âź) 7.0, are accessible under
Precise Infrared Radial Velocities from Keck/NIRSPEC and the Search for Young Planets
We present a high-precision infrared radial velocity study of late-type stars
using spectra obtained with NIRSPEC at the W. M. Keck Observatory. Radial
velocity precisions of 50 m/s are achieved for old field mid-M dwarfs using
telluric features for precise wavelength calibration. Using this technique, 20
young stars in the {\beta} Pic (age ~12 Myr) and TW Hya (age ~8 Myr)
Associations were monitored over several years to search for low mass
companions; we also included the chromospherically active field star GJ 873 (EV
Lac) in this survey. Based on comparisons with previous optical observations of
these young active stars, radial velocity measurements at infrared wavelengths
mitigate the radial velocity noise caused by star spots by a factor of ~3.
Nevertheless, star spot noise is still the dominant source of measurement error
for young stars at 2.3 {\mu}m, and limits the precision to ~77 m/s for the
slowest rotating stars (v sin i < 6 km/s), increasing to ~168 m/s for rapidly
rotating stars (v sin i > 12 km/s). The observations reveal both GJ 3305 and
TWA 23 to be single-lined spectroscopic binaries; in the case of GJ 3305, the
motion is likely caused by its 0.09" companion, identified after this survey
began. The large amplitude, short-timescale variations of TWA 13A are
indicative of a hot Jupiter-like companion, but the available data are
insufficient to confirm this. We label it as a candidate radial velocity
variable. For the remainder of the sample, these observations exclude the
presence of any 'hot' (P < 3 days) companions more massive than 8 MJup, and any
'warm' (P < 30 days) companions more massive than 17 MJup, on average. Assuming
an edge-on orbit for the edge-on disk system AU Mic, these observations exclude
the presence of any hot Jupiters more massive than 1.8 MJup or warm Jupiters
more massive than 3.9 MJup.Comment: Accepted for publication in The Astrophysical Journal. 18 pages, 7
figure
Constrained by managerialism : caring as participation in the voluntary social services
The data in this study show that care is a connective process, underlying and motivating participation and as a force that compels involvement in the lives of others, care is at least a micro-participative process. Care or affinity not only persisted in the face of opposition, but it was also used by workers as a counter discourse and set of practices with which to resist the erosion of worker participation and open up less autonomized practices and ways of connecting with fellow staff, clients and the communities they served. The data suggest that while managerialism and taylorised practice models may remove or reduce opportunities for worker participation, care is a theme or storyline that gave workers other ways to understand their work and why they did it, as well as ways they were prepared to resist managerial priorities and directives, including the erosion of various kinds of direct and indirect participation. The degree of resistance possible, even in the highly technocratic worksite in Australia, shows that cracks and fissures exist within managerialism
The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-ray, and H-alpha Observations
We present the first simultaneous, multi-wavelength observations of an L
dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with
the Very Large Array, the Chandra X-ray Observatory, and the Kitt Peak 4-m
telescope. We detect strongly variable and periodic radio emission (P=3 hr)
with a fraction of about 60% circular polarization. No X-ray emission is
detected to a limit of L_X/L_{bol}<2e-5, several hundred times below the
saturation level observed in early M dwarfs. Similarly, we do not detect
H-alpha emission to a limit of L_{H-alpha}/L_{bol}<2e-7, the deepest for any L
dwarf observed to date. The ratio of radio to X-ray luminosity is at least four
orders of magnitude in excess of that observed in a wide range of active stars
(including M dwarfs) providing the first direct confirmation that late-M and L
dwarfs violate the radio/X-ray correlation. The radio emission is due to
gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which
is maintained on timescales longer than three years. The detected 3-hour period
may be due to (i) the orbital motion of a companion at a separation of about
five stellar radii, similar to the configuration of RS CVn systems, (ii) an
equatorial rotation velocity of about 37 km/s and an anchored, long-lived
magnetic field, or (iii) periodic release of magnetic stresses in the form of
weak flares. In the case of orbital motion, the magnetic activity may be
induced by the companion, possibly explaining the unusual pattern of activity
and the long-lived signal. We conclude that fully convective stars can maintain
a large-scale and stable magnetic field, but the lack of X-ray and H-alpha
emission indicates that the atmospheric conditions are markedly different than
in early-type stars and even M dwarfs. [abridged]Comment: Submitted to ApJ; 26 pages, 15 figure
Diverse soil carbon dynamics expressed at the molecular level
The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change
Magnetically channeled accretion in T Tauri stars : a dynamical process
We review observational evidence and open issues related to the process of
magnetospheric accretion in T Tauri stars. Emphasis is put on recent numerical
simulations and observational results which suggest that the interaction
between the stellar magnetosphere and the inner accretion disk is a highly time
dependent process on timescales ranging from hours to months.Comment: To appear in Open Issues in Local Star Formation and Early Stellar
Evolution, eds. J. Lepine, J. Gregorio-Hete
- âŠ