555 research outputs found
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars
Soft X-ray coronal spectra at low activity levels observed by RESIK
The quiet-Sun X-ray emission is important for deducing coronal heating
mechanisms, but it has not been studied in detail since the Orbiting Solar
Observatory (OSO) spacecraft era. Bragg crystal spectrometer X-ray observations
have generally concentrated on flares and active regions. The high sensitivity
of the RESIK (REntgenovsky Spectrometer s Izognutymi Kristalami) instrument on
the CORONAS-F solar mission has enabled the X-ray emission from the quiet
corona to be studied in a systematic way for the first time. Our aim is to
deduce the physical conditions of the non-flaring corona from RESIK line
intensities in several spectral ranges using both isothermal and multithermal
assumptions. We selected and analyzed spectra in 312 quiet-Sun intervals in
January and February 2003, sorting them into 5 groups according to activity
level. For each group, the fluxes in selected spectral bands have been used to
calculate values parameters for the best-fit that lead to a intensities
characteristic of each group. We used both isothermal and multitemperature
assumptions, the latter described by differential emission measure (DEM)
distributions. RESIK spectra cover the wavelength range (3.3-6.1 A). This
includes emission lines of highly ionized Si, S, Cl, Ar, and K, which are
suitable for evaluating temperature and emission measure, were used. The RESIK
spectra during these intervals of very low solar activity for the first time
provide information on the temperature structure of the quiet corona. Although
most of the emission seems to arise from plasma with a temperature between 2MK
and 3MK, there is also evidence of a hotter plasma (T approx. 10MK) with an
emission measure 3 orders smaller than the cooler component. Neither coronal
nor photospheric element abundances appear to describe the observed spectra
satisfactorily.Comment: Submitting 1 Latex and 7 figure file
Mass Outflow from Red Giant Stars in M13, M15, and M92
Chromospheric model calculations of the Halpha line for selected red giant
branch (RGB) and asymptotic giant branch (AGB) stars in the globular clusters
M13, M15, and M92 are constructed to derive mass loss rates. The model spectra
are compared to the observations obtained with the Hectochelle on the MMT
telescope. These stars show strong Halpha emissions and blue-shifted Halpha
cores signaling that mass outflow is present in all stars. Outflow velocities
of 3-19 km/s, larger than indicated by Halpha profiles, are needed in the upper
chromosphere to achieve good agreement between the model spectra and the
observations. The resulting mass loss rates range from 0.6*10^{-9} to 5*10^{-9}
Msun/yr, which are about an order of magnitude lower than predicted from
"Reimers' law" or inferred from the infrared excess of similar stars. The mass
loss rate increases slightly with luminosity and with decreasing effective
temperature. Stars in the more metal-rich M13 have higher mass loss rates by a
factor of ~2 than in the metal-poor clusters M15 and M92. A fit to the mass
loss rates is given by: M [Msun/yr] = 0.092 * L^{0.16} * Teff^{-2.02} *
A^{0.37} where A=10^[Fe/H]. Multiple observations of stars revealed one object
in M15, K757, in which the mass outflow increased by a factor of 6 between two
observations separated by 18 months. Other stars showed changes in mass loss
rate by a factor of 1.5 or less.Comment: 28 pages, 10 figures, 3 tables, Accepted in Astronomical Journa
VIP21, a 21-kD membrane protein is an integral component of trans-Golgi-network-derived transport vesicles
In simple epithelial cells, apical and basolateral proteins are sorted into separate vesicular carriers before delivery to the appropriate plasma membrane domains. To dissect the putative sorting machinery, we have solubilized Golgi-derived transport vesicles with the detergent CHAPS and shown that an apical marker, influenza haemagglutinin (HA), formed a large complex together with several integral membrane proteins. Remarkably, a similar set of CHAPS-insoluble proteins was found after solubilization of a total cellular membrane fraction. This allowed the cloning of a cDNA encoding one protein of this complex, VIP21 (Vesicular Integral-membrane Protein of 21 kD). The transiently expressed protein appeared on the Golgi-apparatus, the plasma membrane and vesicular structures. We propose that VIP21 is a component of the molecular machinery of vesicular transport
- …