65 research outputs found
Smooth X-ray variability from Ophiuchi A+B. A strongly magnetized primary B2 star?
X-rays from massive stars are ubiquitous yet not clearly understood. In an
XMM-Newton observation devoted to observe the first site of star formation in
the Ophiuchi dark cloud, we detect smoothly variable X-ray emission from
the B2IV+B2V system of Ophiuchi. Tentatively we assign the emission to
the primary component. The light curve of the pn camera shows a first phase of
low, almost steady rate, then a rise phase of duration of 10 ks, followed by a
high rate phase. The variability is seen primarily in the band 1.0-8.0 keV
while little variability is detected below 1 keV. The spectral analysis of the
three phases reveals the presence of a hot component at 3.0 keV that adds up to
two relatively cold components at 0.9 keV and 2.2 keV. We explain the smooth
variability with the emergence of an extended active region on the surface of
the primary star due to its fast rotation (v km/s). We estimate
that the region has diameter in the range R. The hard X-ray
emission and its variability hint a magnetic origin, as suggested for few other
late-Oearly-B type stars. We also discuss an alternative explanation based
on the emergence from occultation of a young (5-10 Myr) low mass companion
bright and hot in X-rays.Comment: 4 pages, 3 figures, 2 tables. Accepted for publication as a letter in
A&
The early B-type star Rho Oph A is an X-ray lighthouse
We present the results of a 140 ks XMM-Newton observation of the B2 star
Ophiuchi A. The star has exhibited strong X-ray variability: a
cusp-shaped increase of rate, similar to that which we partially observed in
2013, and a bright flare. These events are separated in time by about 104 ks,
which likely corresponds to the rotational period of the star (1.2 days). Time
resolved spectroscopy of the X-ray spectra shows that the first event is caused
by an increase of the plasma emission measure, while the second increase of
rate is a major flare with temperatures in excess of 60 MK ( keV).
From the analysis of its rise, we infer a magnetic field of G and a
size of the flaring region of cm, which corresponds
to of the stellar radius. We speculate that either an intrinsic
magnetism that produces a hot spot on its surface or an unknown low mass
companion are the source of such X-rays and variability. A hot spot of magnetic
origin should be a stable structure over a time span of 2.5 years, and
suggests an overall large scale dipolar magnetic field that produces an
extended feature on the stellar surface. In the second scenario, a low mass
unknown companion is the emitter of X-rays and it should orbit extremely close
to the surface of the primary in a locked spin-orbit configuration, almost on
the verge of collapsing onto the primary. As such, the X-ray activity of the
secondary star would be enhanced by its young age, and the tight orbit as in RS
Cvn systems and Ophiuchi would constitute an extreme system that is
worthy of further investigation.Comment: 10 pages, 7 figures, 2 tables, A&A accepted, this is the version
after the language editor correction
No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter
About 20% out of the known exoplanets are Jupiter analogs orbiting
very close to their parent stars. It is still under debate to what detectable
level such hot Jupiters possibly affect the activity of the host stars through
tidal or magnetic star-planet interaction. In this paper we report on an 87 ks
Chandra observation of the hot Jupiter hosting star WASP-18. This system is
composed of an F6 type star and a hot Jupiter of mass orbiting
in less than 20 hr around the parent star. On the basis of an isochrone
fitting, WASP-18 is thought to be 600 Myr old and within the range of
uncertainty of 0.5-2 Gyr. The star is not detected in X-rays down to a
luminosity limit of erg/s, more than two orders of magnitude
lower than expected for a star of this age and mass. This value proves an
unusual lack of activity for a star with estimated age around 600 Myr. We argue
that the massive planet can play a crucial role in disrupting the stellar
magnetic dynamo created within its thin convective layers. Another additional
212 X-ray sources are detected in the Chandra image. We list them and briefly
discuss their nature.Comment: 8 pages, 7 figures, 5 tables. Accepted for publication in A&
The first stars of the Rho Ophiuchi Dark Cloud. XMM-Newton view of Rho Oph and its neighbors
Star formation in molecular clouds can be triggered by the dynamical action
of winds from massive stars. Furthermore, X-ray and UV fluxes from massive
stars can influence the life time of surrounding circumstellar disks. We
present the results of a 53 ks XMM-Newton observation centered on the Rho
Ophiuchi A+B binary system. Rho Ophiiuchi lies in the center of a ring of dust,
likely formed by the action of its winds. This region is different from the
dense core of the cloud (L1688 Core F) where star formation is at work. X-rays
are detected from Rho Ophiuchi as well as a group of surrounding X-ray sources.
We detected 89 X-ray sources, 47 of them have at least one counterpart in 2MASS
+ All-WISE catalogs. Based on IR and X-ray properties, we can distinguish
between young stellar objects (YSOs) belonging to the cloud and background
objects. Among the cloud members, we detect 3 debris disk objects and 22
disk-less / Class III young stars. We show that these stars have ages in
Myr, and are significantly older than the YSOs in L1688. We speculate that they
are the result of an early burst of star formation in the cloud. An X-ray
energy of ergs has been injected into the surrounding
medium during the past Myr, we discuss the effects of such energy budget in
relation to the cloud properties and dynamics.Comment: 17 pages, 9 figures, 7 tables. Accepted for publication to Astronomy
& Astrophysic
Hot Jupiters accreting onto their parent stars: effects on the stellar activity
Hot Jupiters (HJs) are massive gaseous planets orbiting close to their host
stars. Due to their physical characteristics and proximity to the central star,
HJs are the natural laboratories to study the process of star-planet
interaction (SPI). Phenomena related to SPI may include the inflation and the
evaporation of planetary atmospheres, the formation of cometary tails and bow
shocks and magnetospheric interaction between the magnetic field of the planet
and that of the star. Several works suggest that some systems show enhanced
stellar activity in phase with the planetary rotation period. In this work, we
use a 3D magneto-hydrodynamic model that describes a system composed of a star
and an HJ and that includes the corresponding planetary and stellar winds. The
aim is to investigate whether the material evaporating from the planet
interacts with the stellar extended corona, and generates observable features.
Our simulation shows that, in some conditions, the planetary wind expands and
propagates mainly along the planetary orbit. Moreover, part of the planetary
wind collides with the stellar wind and a fraction of the planet's outflow is
funnelled by the stellar magnetic field and hits the stellar surface. In both
events, the material is heated up to temperatures of a few MK by a shock. These
phenomena could manifest in the form of enhanced stellar activity at some
orbital phases of the planet.Comment: Accepted for publication in Astronomische Nachrichten - Astronomical
Note
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