1,992 research outputs found
Pre main sequence: Accretion & Outflows
Low-mass pre-main sequence (PMS) stars are strong X-ray sources, because they
possess hot corona like their older main-sequence counterparts. Unique to young
stars, however, are X-rays from accretion and outflows, and both processes are
of pivotal importance for star and planet formation. We describe how X-ray data
provide important insight into the physics of accretion and outflows. First,
mass accreted from a circumstellar disk onto the stellar surface reaches
velocities up to a few hundred km/s, fast enough to generate soft X-rays in the
post-shock region of the accretion shock. X-ray observations together with
laboratory experiments and numerical simulations show that the accretion
geometry is complex in young stars. Specifically, the center of the accretion
column is likely surrounded by material shielding the inner flow from view but
itself also hot enough to emit X-rays. Second, X-rays are observed in two
locations of protostellar jets: an inner stationary emission component probably
related to outflow collimation and outer components, which evolve withing years
and are likely related to working surfaces where the shock travels through the
jet. Jet-powered X-rays appear to trace the fastest jet component and provide
novel information on jet launching in young stars. We conclude that X-ray data
will continue to be highly important for understanding star and planet
formation, because they directly probe the origin of many emission features
studied in other wavelength regimes. In addition, future X-ray missions will
improve sensitivity and spectral resolution to probe key model parameters (e.g.
velocities) in large samples of PMS stars.Comment: Invited chapter for the "Handbook of X-ray and Gamma-ray
Astrophysics" (Eds. C. Bambi and A. Santangelo, Springer Nature, 2022),
accepted (34 pages, 11 figures
Binary Properties from Cepheid Radial Velocities (CRaV)
We have examined high accuracy radial velocities of Cepheids to determine the
binary frequency. The data are largely from the CORAVEL spectrophotometer and
the Moscow version, with a typical uncertainty of ~km~s, and a
time span from 1 to 20 years. A systemic velocity was obtained by removing the
pulsation component using a high order Fourier series. From this data we have
developed a list of stars showing no orbital velocity larger than
~km~s. The binary fraction was analyzed as a function of
magnitude, and yields an apparent decrease in this fraction for fainter stars.
We interpret this as incompleteness at fainter magnitudes, and derive the
preferred binary fraction of \% ( \% per decade of orbital
period) from the brightest 40 stars. Comparison of this fraction in this period
range (1-20 years) implies a large fraction for the full period range. This is
reasonable in that the high accuracy velocities are sensitive to the longer
periods and smaller orbital velocity amplitudes in the period range sampled
here. Thus the Cepheid velocity sample provides a sensitive detection in the
period range between short period spectroscopic binaries and resolved
companions. The recent identification of Cep as a binary with very low
amplitude and high eccentricity underscores the fact that the binary fractions
we derive are lower limits, to which other low amplitude systems will probably
be added. The mass ratio (q) distribution derived from ultraviolet observations
of the secondary is consistent with a flat distribution for the applicable
period range (1 to 20 years).Comment: accepted for publication in A
The Mass of the Cepheid V350 Sgr
V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot
companion which is prominent in the ultraviolet and which is not itself a
binary. We have obtained two high resolution echelle spectra of the companion
at orbital velocity maximum and minimum with the Hubble Space Telescope (HST)
Space Telescope Imaging Spectrograph (STIS) in the 1320 to 1510 \AA\/ region.
By cross-correlating these spectra we obtained the orbital velocity amplitude
of the companion with an uncertainty in the companion amplitude of 1.9 km
sec. This provides a mass ratio of the Cepheid to the companion of 2.1.
The ultraviolet energy distribution of the companion provides the mass of the
companion, yielding a Cepheid mass of 5.2 0.3 M. This mass
requires some combination of moderate main sequence core convective overshoot
and rotation to match evolutionary tracks.Comment: Accepted by Ap
Recommended from our members
SPION@polydehydroalanine hybrid particles
It is generally accepted that a protein corona is rapidly formed upon exposure of nanoparticles to biological fluids and that both the amount and the composition of adsorbed proteins affect the dispersion properties of the resulting particles. Hereby, the net charge and overall charge density of the pristine nanoparticles are supposed to play a crucial role. In an attempt to control both charge and charge distribution, we report on the coating of superparamagnetic iron oxide nanoparticles (SPIONs) with different polyelectrolytes. Starting from orthogonally protected polydehydroalanine, the material can be easily transformed into a polyanion (poly(tert-butoxycarbonyl acrylic acid), PtBAA), polycation (poly(aminomethylacrylate), PAMA), or even a polyzwitterion (polydehydroalanine, PDha). While coating of SPIONs with PtBAA and PDha was shown to be successful, approaches using PAMA have failed so far. The dispersion properties of the resulting hybrid particles have been investigated using dynamic light scattering (DLS), zeta-potential, and TEM measurements – the amount of adsorbed polymer was quantified using vibrating sample magnetometry (VSM) and thermogravimetric analysis (TGA)
YSOVAR: Mid-infrared Variability among YSOs in the Star Formation Region Serpens South
We present a time-variability study of young stellar objects (YSOs) in the Serpens South cluster performed at 3.6 and 4.5 μm with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project. We have collected light curves for more than 1500 sources, including 85 cluster members, over 38 days. This includes 44 class I sources, 19 sources with flat spectral energy distributions (SEDs), 17 class II sources, and five diskless YSO candidates. We find a high variability fraction among embedded cluster members of ~70%, whereas young stars without a detectable disk display no variability. We detect periodic variability for 32 sources with periods primarily in the range of 0.2–14 days and a subset of fast rotators thought to be field binaries. The timescale for brightness changes are shortest for stars with the most photospheric SEDs and longest for those with flat or rising SEDs. While most variable YSOs become redder when fainter, as would be expected from variable extinction, about 10% get bluer as they get fainter. One source, SSTYSV J183006.13−020108.0, exhibits "cyclical" color changes
X-Ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-mass Companion
V473 Lyr is a classical Cepheid which is unique in having substantial
amplitude variations with a period of approximately 3.3 years, thought to be
similar to the Blazhko variations in RR Lyrae stars. We obtained an {\it
XMM-Newton} observation of this star to followup a previous detection in
X-rays. Rather than the X-ray burst and rapid decline near maximum radius seen
in Cephei itself, the X-ray flux in V473 Lyr remained constant for a
third of the pulsation cycle covered by the observation. Thus the X-rays are
most probably not produced by the changes around the pulsation cycle. The X-ray
spectrum is soft (kT = 0.6 keV), with
X-ray properties which are consistent with a young low mass companion.
Previously there was no evidence of a companion in radial velocities or in {\it
Gaia} and {\it Hipparcos} proper motions. While this rules out companions which
are very close or very distant, a binary companion at a separation between 30
and 300 AU is possible. This is an example of an X-ray observation revealing
evidence of a low mass companion, which is important in completing the mass
ratio statistics of binary Cepheids. Furthermore, the detection of a young
X-ray bright companion is a further indication that the Cepheid (primary) is a
Population I star, even though its pulsation behavior differs from other
classical Cepheids.Comment: Accepted by A
A small satellite version of a soft x-ray polarimeter
We describe a new implementation of a broad-band soft X-ray polarimeter, substantially based on a previous design. This implementation, the Pioneer Soft X-ray Polarimeter (PiSoX) is a SmallSat, designed for NASA’s call for Astrophysics Pioneers, small missions that could be CubeSats, balloon experiments, or SmallSats. As in REDSoX, the grating arrangement is designed optimally for the purpose of polarimetry with broad-band focussing optics by matching the dispersion of the spectrometer channels to laterally graded multilayers (LGMLs). The system can achieve polarization modulation factors over 90%. For PiSoX, the optics are lightweight Si mirrors in a one-bounce parabolic configuration. High efficiency, blazed gratings from opposite sectors are oriented to disperse to a LGML forming a channel covering the wavelength range from 35 Å to 75 Å (165 - 350 eV). Upon satellite rotation, the intensities of the dispersed spectra, after reflection and polarizing by the LGMLs, give the three Stokes parameters needed to determine a source’s linear polarization fraction and orientation. The design can be extended to higher energies as LGMLs are developed further. We describe examples of the potential scientific return from instruments based on this design
YSOVAR: Mid-infrared Variability among YSOs in the Star Formation Region Serpens South
We present a time-variability study of young stellar objects (YSOs) in the Serpens South cluster performed at 3.6 and 4.5 μm with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project. We have collected light curves for more than 1500 sources, including 85 cluster members, over 38 days. This includes 44 class I sources, 19 sources with flat spectral energy distributions (SEDs), 17 class II sources, and five diskless YSO candidates. We find a high variability fraction among embedded cluster members of ~70%, whereas young stars without a detectable disk display no variability. We detect periodic variability for 32 sources with periods primarily in the range of 0.2–14 days and a subset of fast rotators thought to be field binaries. The timescale for brightness changes are shortest for stars with the most photospheric SEDs and longest for those with flat or rising SEDs. While most variable YSOs become redder when fainter, as would be expected from variable extinction, about 10% get bluer as they get fainter. One source, SSTYSV J183006.13−020108.0, exhibits "cyclical" color changes
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