396 research outputs found
The X-ray eclipse of OY Car resolved with XMM-Newton: X-ray emission from the polar regions of the white dwarf
We present the XMM-Newton X-ray eclipse lightcurve of the dwarf nova OY Car.
The eclipse ingress and egress are well resolved for the first time in any
dwarf nova placing strong constraints on the size and location of the X-ray
emitting region. We find good fits to a simple linear eclipse model, giving
ingress/egress durations of 30+/-3 sec. Remarkably this is shorter than the
ingress/egress duration of the sharp eclipse in the optical as measured by Wood
et al. (1989) and ascribed to the white dwarf (43+/-2 sec). We also find that
the X-ray eclipse is narrower than the optical eclipse by 14+/-2 sec, which is
precisely the difference required to align the second and third contact points
of the X-ray and optical eclipses. We discuss these results and conclude that
X-ray emission in OY Car most likely arises from the polar regions of the white
dwarf.
Our data were originally reported by Ramsay et al (2001), but they did not
make a quantitative measurement of eclipse parameters. We have also corrected
important timing anomalies present in the data available at that time.Comment: 6 pages, 5 figures; accepted for publication in MNRA
ROSAT observations of V471 Tauri, showing that stellar activity is determined by rotation, not age
I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471
Tauri. The hard X-ray emission (>0.4keV) is not eclipsed by the K star,
demonstrating conclusively that this component cannot be emitted by the white
dwarf. Instead I show that its spectrum and luminosity are consistent with
coronal emission from the tidally spun-up K star. The star is more active than
other K stars in the Hyades, but equally active as K stars in the Pleiades with
the same rotation periods, demonstrating that rotation--and not age--is the key
parameter in determining the level of stellar activity.
The soft X-ray emission (<0.4keV) is emitted predominately by the white dwarf
and is modulated on its spin period. I find that the pulse-profile is stable on
timescales of hours and years, supporting the idea that it is caused by opacity
of accreted material. The profile itself shows that the magnetic field
configuration of the white dwarf is dipolar and that the magnetic axis passes
through the centre of the star.
There is an absorption feature in the lightcurve of the white dwarf, which
occurs at a time when our line-of-sight passes within a stellar radius of the K
star. The column density and duration of this feature imply a volume and mass
for the absorber which are similiar to those of coronal mass ejections of the
Sun.Comment: 7 pages, 7 figures, accepted for publication in MNRAS. Also available
at http://www.star.le.ac.uk/~pjw
Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an
ultracool dwarf star recently discovered to host three transiting and temperate
Earth-sized planets. We find the star is a relatively strong and variable
coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun,
despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with
the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation
of the planets that is many times stronger than experienced by the present-day
Earth. Using a simple energy-limited model we show that the relatively close-in
Earth-sized planets, which span the classical habitable zone of the star, are
subject to sufficient X-ray and EUV irradiation to significantly alter their
primary and any secondary atmospheres. Understanding whether this high-energy
irradiation makes the planets more or less habitable is a complex question, but
our measured fluxes will be an important input to the necessary models of
atmospheric evolution.Comment: 5 pages, published as a letter in MNRAS (accepted 16 September 2016
A precise optical transmission spectrum of the inflated exoplanet WASP-52b
We have measured a precise optical transmission spectrum for WASP-52b, a
highly inflated hot Jupiter with an equilibrium temperature of 1300 K. Two
transits of the planet were observed spectroscopically at low resolution with
the auxiliary-port camera (ACAM) on the William Herschel Telescope (WHT),
covering a wide range of 4000-8750 \AA. We use a Gaussian process approach to
model the correlated noise in the multi-wavelength light curves, resulting in a
high precision relative transmission spectrum with errors on the order of a
pressure scale height. We attempted to fit a variety of different
representative model atmospheres to the transmission spectrum, but did not find
a satisfactory match to the entire spectral range. For the majority of the
covered wavelength range (4000-7750 \AA) the spectrum is flat, and can be
explained by an optically thick and grey cloud layer at 0.1 mbar, but this is
inconsistent with a slightly deeper transit at wavelengths \AA. We
were not able to find an obvious systematic source for this feature, so this
opacity may be the result of an additional unknown absorber.Comment: Submitted to MNRAS 17 Jan 2017, revised version after comments from
reviewer, 12 pages, 10 figure
Single site observations of \textit{TESS} single transit detections
Context: TESS has been successfully launched and has begin data acquisition.
To expedite the science that may be performed with the resulting data it is
necessary to gain a good understanding of planetary yields. Given the observing
strategy employed by TESS the probability of detecting single transits in long
period systems is increased. These systems require careful consideration.
Aims: To simulate the number of TESS transit detections during its 2 year
mission with a particular emphasis on single transits. Additionally, to
determine the feasibility of ground-based follow-up observations from a single
site.
Methods: A distribution of planets is simulated around the 4 million
stars in the TESS Candidate Target List. These planets are tested for
detectable transits and characterised. Based on simulated parameters the single
transit detections are further analysed to determine which are amenable to
ground-based follow-up.
Results: TESS will discover an approximate lower bound of 4700 planets with
around 460 being single transits. A large fraction of these will be observable
from a single ground-based site. This paper finds that, in a single year,
approximately 1000 transit events of around 320 unique TESS single transit
detections are theoretically observable.
Conclusions: As we consider longer period exoplanets the need for exploring
single transit detections increases. For periods days the number of
single transit detections outnumber multitransits by a factor of 3 (8218
and 257 respectively) a factor which only grows as longer period
detections are considered. Therefore, it is worth expending the extra effort
required to follow-up these more challenging, but potentially very rewarding,
discoveries. Additionally, we conclude that a large fraction of these targets
can be theoretically observed from just a single ground-based site.Comment: 12 pages, 19 figures. To be published in Astronomy and Astrophysic
The shared evaporation history of three sub-Neptunes spanning the radius-period valley of a Hyades star
We model the evaporation histories of the three planets around K2-136, a
K-dwarf in the Hyades open cluster with an age of 700 Myr. The star hosts three
transiting planets, with radii of 1.0, 3.0 and 1.5 Earth radii, where the
middle planet lies above the radius-period valley and the inner and outer
planets are below. We use an XMM-Newton observation to measure the XUV
radiation environment of the planets, finding that the X-ray activity of K2-136
is lower than predicted by models but typical of similar Hyades members. We
estimate the internal structure of each planet, and model their evaporation
histories using a range of structure and atmospheric escape formulations. While
the precise X-ray irradiation history of the system may be uncertain, we
exploit the fact that the three planets must have shared the same history. We
find that the Earth-sized K2-136b is most likely rocky, with any primordial
gaseous envelope being lost within a few Myr. The sub-Neptune, K2-136c, has an
envelope contributing 1-1.7% of its mass that is stable against evaporation
thanks to the high mass of its rocky core, whilst the super-Earth, K2-136d,
must have a mass at the upper end of the allowed range in order to retain any
of its envelope. Our results are consistent with all three planets beginning as
sub-Neptunes that have since been sculpted by atmospheric evaporation to their
current states, stripping the envelope from planet b and removing most from
planet d whilst preserving planet c above the radius-period valley.Comment: Accepted for publication on MNRA
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