27 research outputs found
On the late spectral types of cataclysmic variable secondaries
We investigate why the spectral type of most cataclysmic variable (CV)
secondaries is significantly later than that of a ZAMS star with the same mean
density. Using improved stellar input physics, tested against observations of
low-mass stars at the bottom of the main sequence, we calculate the secular
evolution of CVs with low-mass donors. We consider sequences with different
mass transfer rates and with a different degree of nuclear evolution of the
donor prior to mass transfer. Systems near the upper edge of the gap ( h) can be reproduced by models with a wide range of mass transfer rates
from 1.5 \times 10^{-9} \msolyr to 10^{-8} \msolyr. Evolutionary sequences
with a small transfer rate and donors that are substantially evolved off the
ZAMS (central hydrogen content ) reproduce CVs with late spectral
types above P \simgr 6 h. Systems with the most discrepant (late) spectral
type should have the smallest donor mass at any given . Consistency with the
period gap suggests that the mass transfer rate increases with decreasing donor
mass for evolved sequences above the period gap. In this case, a
single-parameter family of sequences with varying \xc and increasing mass
transfer rate reproduces the full range of observed spectral types. This would
imply that CVs with such evolved secondaries dominate the CV population.Comment: 9 pages, Latex file, uses mn.sty, accepted for publication in MNRA
How do psychiatrists address delusions in first meetings in acute care? A qualitative study
Licensed under the Creative Commons Attribution Licens
Supernova Kicks, Magnetic Braking, and Neutron-Star Binaries
We consider the formation of low-mass X-ray binaries containing accreting
neutron stars via the helium-star supernova channel. The predicted relative
number of short-period transients provides a sensitive test of the input
physics in this process. We investigate the effect of varying mean kick
velocities, orbital angular momentum loss efficiencies, and common envelope
ejection efficiencies on the subpopulation of short-period systems, both
transient and persistent. Guided by the thermal-viscous disk instability model
in irradiation-dominated disks, we posit that short-period transients have
donors close to the end of core-hydrogen burning. We find that with increasing
mean kick velocity the overall short-period fraction, s, grows, while the
fraction, r, of systems with evolved donors among short-period systems drops.
This effect, acting in opposite directions on these two fractions, allows us to
constrain models of LMXB formation through comparison with observational
estimates of s and r. Without fine tuning or extreme assumptions about
evolutionary parameters, consistency between models and current observations is
achieved for a regime of intermediate average kick magnitudes of about 100-200
km/s, provided that (i) orbital braking for systems with donor masses in the
range 1-1.5 solar masses is weak, i.e., much less effective than a simple
extrapolation of standard magnetic braking beyond 1.0 solar mass would suggest,
and (ii) the efficiency of common envelope ejection is low.Comment: 24 pages, AAATeX, accepted for publication in The Astrophysical
Journa
Description of the Scenario Machine
We present here an updated description of the "Scenario Machine" code. This
tool is used to carry out a population synthesis of binary stars. Previous
version of the description can be found at
http://xray.sai.msu.ru/~mystery//articles/review/contents.htmlComment: 32 pages, 3 figures. Corrected typo
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
In-flight radiometric calibration of the Metis Visible Light channel using stars and comparison with STEREO-A/COR2 data
Context. We present the results for the in-flight radiometric calibration performed for the Visible Light (VL) channel of the Metis coronagraph on board Solar Orbiter. Aims. The radiometric calibration is a fundamental step in building the official pipeline of the instrument, devoted to producing the calibrated data in physical units (L2 data). Methods. To obtain the radiometric calibration factor (ĂÎŒVL), we used stellar targets transiting the Metis field of view. We derived ĂÎŒVLby determining the signal of each calibration star by means of the aperture photometry and calculating its expected flux in the Metis band pass. The analyzed data set covers the time range from the beginning of the Cruise Phase of the mission (June 2020) until March 2021. Results. Considering the uncertainties, the estimated factor ĂÎŒVLis in a good agreement with that obtained during the on-ground calibration campaign. This implies that up to March 2021 there was no measurable reduction in the VL channel throughput. Finally, we compared the total and polarized brightness visible light images of the solar corona acquired with Metis and STEREO-A/COR2 during the November 2020 superior conjunction of these instruments. A general good agreement was obtained between the images of these instruments for both the total and polarized brightness
Coronal Heating Rate in the Slow Solar Wind
This Letter reports the first observational estimate of the heating rate in the slowly expanding solar corona. The analysis exploits the simultaneous remote and local observations of the same coronal plasma volume, with the Solar Orbiter/Metis and the Parker Solar Probe instruments, respectively, and relies on the basic solar wind magnetohydrodynamic equations. As expected, energy losses are a minor fraction of the solar wind energy flux, since most of the energy dissipation that feeds the heating and acceleration of the coronal flow occurs much closer to the Sun than the heights probed in the present study, which range from 6.3 to 13.3 R & ODOT;. The energy deposited to the supersonic wind is then used to explain the observed slight residual wind acceleration and to maintain the plasma in a nonadiabatic state. As derived in the Wentzel-Kramers-Brillouin limit, the present energy transfer rate estimates provide a lower limit, which can be very useful in refining the turbulence-based modeling of coronal heating and subsequent solar wind acceleration
Exploring the Solar Wind from Its Source on the Corona into the Inner Heliosphere during the First Solar Orbiter-Parker Solar Probe Quadrature
This Letter addresses the first Solar Orbiter (SO)âParker Solar Probe (PSP) quadrature, occurring on 2021 January 18 to investigate the evolution of solar wind from the extended corona to the inner heliosphere. Assuming ballistic propagation, the same plasma volume observed remotely in the corona at altitudes between 3.5 and 6.3 solar radii above the solar limb with the Metis coronagraph on SO can be tracked to PSP, orbiting at 0.1 au, thus allowing the local properties of the solar wind to be linked to the coronal source region from where it originated. Thanks to the close approach of PSP to the Sun and the simultaneous Metis observation of the solar corona, the flow-aligned magnetic field and the bulk kinetic energy flux density can be empirically inferred along the coronal current sheet with an unprecedented accuracy, allowing in particular estimation of the AlfvĂ©n radius at 8.7 solar radii during the time of this event. This is thus the very first study of the same solar wind plasma as it expands from the sub-AlfvĂ©nic solar corona to just above the AlfvĂ©n surface
First light observations of the solar wind in the outer corona with the Metis coronagraph
In this work, we present an investigation of the wind in the solar corona that has been initiated by observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, designed to measure the wind outflow speed by applying Doppler dimming diagnostics. Metis on Solar Orbiter complements the UVCS spectroscopic observations that were performed during solar activity cycle 23 by simultaneously imaging the polarized visible light and the H?» I Lyman-α corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The Metis observations, taken on May 15, 2020, provide the first HI Lyman-α images of the extended corona and the first instantaneous map of the speed of the coronal plasma outflows during the minimum of solar activity and allow us to identify the layer where the slow wind flow is observed. The polarized visible light (580-640 nm) and the ultraviolet HI Lyα (121.6 nm) coronal emissions, obtained with the two Metis channels, were combined in order to measure the dimming of the UV emission relative to a static corona. This effect is caused by the outward motion of the coronal plasma along the direction of incidence of the chromospheric photons on the coronal neutral hydrogen. The plasma outflow velocity was then derived as a function of the measured Doppler dimming. The static corona UV emission was simulated on the basis of the plasma electron density inferred from the polarized visible light. This study leads to the identification, in the velocity maps of the solar corona, of the high-density layer about ±10° wide, centered on the extension of a quiet equatorial streamer present at the east limb - the coronal origin of the heliospheric current sheet - where the slowest wind flows at about 160 ± 18 km s-1 from 4 Râ to 6 Râ. Beyond the boundaries of the high-density layer, the wind velocity rapidly increases, marking the transition between slow and fast wind in the corona
The Space Weather X-Ray spectrometer for the Helianthus sub-L1 mission with solar photonic propulsion
Helianthus is a phase A study of a space weather station with solar photonic propulsion. The scientific payload will be made of: an X-ray spectrometer to detect solar flares; SailCor, a coronagraph with a wide field of view; a plasma analyzer; a magnetometer. The maximum allowed mass for the entire scientific payload shall not exceed 5 kg. The two imaging devices (coronagraph and X-ray spectrometer) are of fundamental importance for the sake of remotely and timely mapping the status of the Sun and provide Earth stations with early warning of potentially disruptive events. An extensive research on available X-Ray detectors was performed and the Amptek FAST-SDD spectrometer was selected. It is a very light, compact and vacuum compatible instrument. In order to prove the device readiness for flight, a measurement campaign was organized to investigate its performance in terms of spectral range, spectral resolution, dynamic range and response speed. The campaign was run at the INAF XACT facility in Palermo (Italy). This paper describes the facility, the measurement campaign and the results