31 research outputs found
Magnetic Flares on Asymptotic Giant Branch Stars
We investigate the consequences of magnetic flares on the surface of
asymptotic giant branch (AGB) and similar stars. In contrast to the solar wind,
in the winds of AGB stars the gas cooling time is much shorter than the outflow
time. As a result, we predict that energetic flaring will not inhibit, and may
even enhance, dust formation around AGB stars. If magnetic flares do occur
around such stars, we expect some AGB stars to exhibit X-ray emission; indeed
certain systems including AGB stars, such as Mira, have been detected in
X-rays. However, in these cases, it is difficult to distinguish between
potential AGB star X-ray emission and, e.g., X-ray emission from the vicinity
of a binary companion. Analysis of an archival ROSAT X-ray spectrum of the Mira
system suggests an intrinsic X-ray luminosity 2x10^{29} erg/sec and temperature
10^7 K. These modeling results suggest that magnetic activity, either on the
AGB star (Mira A) or on its nearby companion (Mira B), is the source of the
X-rays, but do not rule out the possibility that the X-rays are generated by an
accretion disk around Mira B.Comment: ApJ, Accepted; revised version of astro-ph/020923
Solar-Like Cycle in Asymptotic Giant Branch Stars
I propose that the mechanism behind the formation of concentric semi-periodic
shells found in several planetary nebulae (PNs) and proto-PNs, and around one
asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in
the progenitor AGB stars. The time intervals between consecutive ejection
events is about 200-1,000 years, which is assumed to be the cycle period (the
full magnetic cycle can be twice as long, as is the 22-year period in the sun).
The magnetic field has no dynamical effects; it regulates the mass loss rate by
the formation of magnetic cool spots. The enhanced magnetic activity at the
cycle maximum results in more magnetic cool spots, which facilitate the
formation of dust, hence increasing the mass loss rate. The strong magnetic
activity implies that the AGB star is spun up by a companion, via a tidal or
common envelope interaction. The strong interaction with a stellar companion
explains the observations that the concentric semi-periodic shells are found
mainly in bipolar PNs.Comment: 10 pages, submitted to Ap
A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
FUSE ultraviolet spectra of 8 giant and supergiant stars reveal that high
temperature (3 X 10^5 K) atmospheres are common in luminous cool stars and
extend across the color-magnitude diagram from Alpha Car (F0 II) to the cool
giant Alpha Tau (K5 III). Emission present in these spectra includes
chromospheric H-Lyman Beta, Fe II, C I, and transition region lines of C III, O
VI, Si III, Si IV. Emission lines of Fe XVIII and Fe XIX signaling temperatures
of ~10^7 K and coronal material are found in the most active stars, Beta Cet
and 31 Com. A short-term flux variation, perhaps a flare, was detected in Beta
Cet during our observation. Stellar surface fluxes of the emission of C III and
O VI are correlated and decrease rapidly towards the cooler stars, reminiscent
of the decay of magnetically-heated atmospheres. Profiles of the C III (977A)
lines suggest that mass outflow is underway at T~80,000 K, and the winds are
warm. Indications of outflow at higher temperatures (3 X 10^5K) are revealed by
O VI asymmetries and the line widths themselves. High temperature species are
absent in the M-supergiant Alpha Ori. Narrow fluorescent lines of Fe II appear
in the spectra of many giants and supergiants, apparently pumped by H Lyman
Alpha, and formed in extended atmospheres. Instrumental characteristics that
affect cool star spectra are discussed.Comment: Accept for publication in The Astrophysical Journal; 22 pages of
text, 23 figures and 8 table
A ROSAT Survey of Contact Binary Stars
Contact binary stars are common variable stars which are all believed to emit
relatively large fluxes of x-rays. In this work we combine a large new sample
of contact binary stars derived from the ROTSE-I telescope with x-ray data from
the ROSAT All-Sky Survey (RASS) to estimate the x-ray volume emissivity of
contact binary stars in the galaxy. We obtained x-ray fluxes for 140 contact
binaries from the RASS, as well as 2 additional stars observed by the
XMM-Newton observatory. From these data we confirm the emission of x-rays from
all contact binary systems, with typical luminosities of approximately 1.0 x
10^30 erg s^-1. Combining calculated luminosities with an estimated contact
binary space density, we find that contact binaries do not have strong enough
x-ray emission to account for a significant portion of the galactic x-ray
background.Comment: 19 pages, 5 figures, accepted by A
Hydrodynamical simulations of the jet in the symbiotic star MWC 560 III. Application to X-ray jets in symbiotic stars
In papers I and II in this series, we presented hydrodynamical simulations of
jet models with parameters representative of the symbiotic system MWC 560.
These were simulations of a pulsed, initially underdense jet in a high density
ambient medium. Since the pulsed emission of the jet creates internal shocks
and since the jet velocity is very high, the jet bow shock and the internal
shocks are heated to high temperatures and should therefore emit X-ray
radiation. In this paper, we investigate in detail the X-ray properties of the
jets in our models. We have focused our study on the total X-ray luminosity and
its temporal variability, the resulting spectra and the spatial distribution of
the emission. Temperature and density maps from our hydrodynamical simulations
with radiative cooling presented in the second paper are used together with
emissivities calculated with the atomic database ATOMDB. The jets in our models
show extended and variable X-ray emission which can be characterized as a sum
of hot and warm components with temperatures that are consistent with
observations of CH Cyg and R Aqr. The X-ray spectra of our model jets show
emission line features which correspond to observed features in the spectra of
CH Cyg. The innermost parts of our pulsed jets show iron line emission in the
6.4 - 6.7 keV range which may explain such emission from the central source in
R Aqr. We conclude that MWC 560 should be detectable with Chandra or
XMM-Newton, and such X-ray observations will provide crucial for understanding
jets in symbiotic stars.Comment: 10 pages, 12 figures, accepted for publication in ApJ, uses
emulateap
Some Like it Hot: The X-Ray Emission of The Giant Star YY Mensae
(Abridged abstract) We present an analysis of the X-ray emission of the
rapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton.
Although no obvious flare was detected, the X-ray luminosity changed by a
factor of two between the XMM-Newton and Chandra observations taken 4 months
apart. The coronal abundances and the emission measure distribution have been
derived from three different methods using optically thin collisional
ionization equilibrium models. The abundances show an inverse first ionization
potential (FIP) effect. We further find a high N abundance which we interpret
as a signature of material processed in the CNO cycle. The corona is dominated
by a very high temperature (20-40 MK) plasma, which places YY Men among the
magnetically active stars with the hottest coronae. Lower temperature plasma
also coexists, albeit with much lower emission measure. Line broadening is
reported, which we interpret as Doppler thermal broadening, although rotational
broadening due to X-ray emitting material high above the surface could be
present as well. We use two different formalisms to discuss the shape of the
emission measure distribution. The first one infers the properties of coronal
loops, whereas the second formalism uses flares as a statistical ensemble. We
find that most of the loops in the corona of YY Men have their maximum
temperature equal to or slightly larger than about 30 MK. We also find that
small flares could contribute significantly to the coronal heating in YY Men.
Although there is no evidence of flare variability in the X-ray light curves,
we argue that YY Men's distance and X-ray brightness does not allow us to
detect flares with peak luminosities Lx <= 10^{31} erg/s with current
detectors.Comment: Accepted paper to appear in Astrophysical Journal, issue Nov 10, 2004
(v615). This a revised version. Small typos are corrected. Figure 7 and its
caption and some related text in Sct 7.2 are changed, without incidence for
the conclusion
Outer jet X-ray and radio emission in R Aquarii: 1999.8 to 2004.0
Chandra and VLA observations of the symbiotic star R Aqr in 2004 reveal
significant changes over the three to four year interval between these
observations and previous observations taken with the VLA in 1999 and with
Chandra in 2000. This paper reports on the evolution of the outer thermal X-ray
lobe-jets and radio jets. The emission from the outer X-ray lobe-jets lies
farther away from the central binary than the outer radio jets, and comes from
material interpreted as being shock heated to ~10^6 K, a likely result of
collision between high speed material ejected from the central binary and
regions of enhanced gas density. Between 2000 and 2004, the Northeast (NE)
outer X-ray lobe-jet moved out away from the central binary, with an apparent
projected motion of ~580 km s^-1. The Southwest (SW) outer X-ray lobe-jet
almost disappeared between 2000 and 2004, presumably due to adiabatic expansion
and cooling. The NE radio bright spot also moved away from the central binary
between 2000 and 2004, but with a smaller apparent velocity than of the NE
X-ray bright spot. The SW outer lobe-jet was not detected in the radio in
either 1999 or 2004. The density and mass of the X-ray emitting material is
estimated. Cooling times, shock speeds, pressure and confinement are discussed.Comment: 23 pages, 8 figure
The X-ray R Aquarii: A Two-sided Jet and Central Source
We report Chandra ACIS-S3 x-ray imaging and spectroscopy of the R Aquarii
binary system that show a spatially resolved two-sided jet and an unresolved
central source. This is the first published report of such an x-ray jet seen in
an evolved stellar system comprised of ~2-3 solar masses. At E < 1 keV, the
x-ray jet extends both to the northeast and southwest relative to the central
binary system. At 1 < E < 7.1 keV, R Aqr is a point-like source centered on the
star system. While both 3.5-cm radio continuum emission and x-ray emission
appear coincident in projection and have maximum intensities at ~7.5" northeast
of the central binary system, the next strongest x-ray component is located
\~30" southwest of the central binary system and has no radio continuum
counterpart. The x-ray jets are likely shock heated in the recent past, and are
not in thermal equilibrium. The strongest southwest x-ray jet component may
have been shocked recently since there is no relic radio emission as expected
from an older shock. At the position of the central binary, we detect x-ray
emission below 1.6 keV consistent with blackbody emission at T ~2 x 10^6 K. At
the central star there is also a prominent 6.4 keV feature, a possible
fluorescence or collisionally excited Fe K-alpha line from an accretion disk or
from the wind of the giant star. For this excitation to occur, there must be an
unseen hard source of x-rays or particles in the immediate vicinity of the hot
star. Such a source would be hidden from view by the surrounding edge-on
accretion disk.Comment: PS, 20 pages, including 3 figures PNG, JPG - accepted for publication
in ApJ Letters. Subject headings: stars: individual (R Aquarii) -- binaries:
symbiotic -- circumstellar matter -- stars: white dwarfs -- stars: winds,
outflows -- radio continuum: stars -- x-rays: genera
X-rays from the Mira AB Binary System
We present the results of XMM-Newton X-ray observations of the Mira AB binary
system, which consists of a pulsating, asymptotic giant branch primary and
nearby (~0.6'' separation) secondary of uncertain nature. The EPIC CCD (MOS and
pn) X-ray spectra of Mira AB are relatively soft, peaking at ~1 keV, with only
very weak emission at energies > 3 keV; lines of Ne IX, Ne X, and O VIII are
apparent. Spectral modeling indicates a characteristic temperature T_X ~ 10^7 K
and intrinsic luminosity L_X ~ 5x10^29 erg s^{-1}, and suggests enhanced
abundances of O and, possibly, Ne and Si in the X-ray-emitting plasma. Overall,
the X-ray spectrum and luminosity of the Mira AB system more closely resemble
those of late-type, pre-main sequence stars or late-type, magnetically active
main sequence stars than those of accreting white dwarfs. We conclude that Mira
B is most likely a late-type, magnetically active, main-sequence dwarf, and
that X-rays from the Mira AB system arise either from magnetospheric accretion
of wind material from Mira A onto Mira B, or from coronal activity associated
with Mira B itself, as a consequence of accretion-driven spin-up. One (or both)
of these mechanisms also could be responsible for the recently discovered,
point-like X-ray sources within planetary nebulae.Comment: 14 pages, 3 figures; to appear in the Astrophysical Journa
Candidates for detecting exoplanetary radio emissions generated by magnetosphere-ionosphere coupling
In this paper we consider the magnetosphere-ionosphere (M-I) coupling at
Jupiter-like exoplanets with internal plasma sources such as volcanic moons,
and we have determined the best candidates for detection of these radio
emissions by estimating the maximum spectral flux density expected from planets
orbiting stars within 25 pc using data listed in the NASA/IPAC/NExScI Star and
Exoplanet Database (NStED). In total we identify 91 potential targets, of which
40 already host planets and 51 have stellar X-ray luminosity 100 times the
solar value. In general, we find that stronger planetary field strength,
combined with faster rotation rate, higher stellar XUV luminosity, and lower
stellar wind dynamic pressure results in higher radio power. The top two
targets for each category are Eri and HIP 85523, and CPD-28 332 and
FF And.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Letter