89 research outputs found
The microlensing signatures of starspots
Point lens microlensing events with impact parameter close to the source
stellar radius allow the observer to study the surface brightness profile of
the lensed source. We have examined the effect of photospheric star spots on
multicolour microlensing lightcurves and investigated the detectability of such
spots in different wavebands as a function of spot temperature, position,
radius and lens trajectories. We include the effect of limb darkening and spot
projection as a function of position on the stellar disc. In particular we
apply the updated, state-of-the-art NEXTGEN stellar atmosphere models of
Hauschildt et al which predict very strong limb darkening and which are likely
to be applicable to the source stars considered here. Our results indicate that
star spots generally give a clear signature for transit events. Moreover, this
signature is strongly suppressed by limb darkening for spots close to the limb,
although the spots may still be clearly detected for favourable lens
trajectories. It is also clear that intensive temporal sampling throughout the
duration of the transit is necessary in order for such events to be effective
as a tool for imaging stellar photospheres. Nonetheless, with sufficiently well
sampled light curves of good photometric precision, microlensing can indeed
place useful constraints on the presence or otherwise of photospheric
starspots.Comment: 12 pages, 10 figures, MNRAS 335, 539 (2002
An Unexplained 10 Degree - 40 Degree Shift in the Location of Some Diverse Neutral Atom Data at 1 AU
Four different data sets pertaining to the neutral atom environment at 1 AU
are presented and discussed. These data sets include neutral solar wind and
interstellar neutral atom data from IMAGE/LENA, energetic hydrogen atom data
from SOHO/HSTOF and plasma wave data from the magnetometer on ISEE-3.
Surprisingly, these data sets are centered between 262 degrees and 292 degrees
ecliptic longitude, about 10 degrees - 40 degrees from the upstream
interstellar neutral flow direction at 254 degrees resulting from the motion of
the Sun relative to the local interstellar cloud. Some possible explanations
for this offset, none of which is completely satisfactory, are discussed.Comment: 6 pages, 6 figures, 2 color peer-reviewed paper, in press, COSPAR/WS
A simple model for the evolution of disc galaxies: The Milky Way
A simple model for the evolution of disc galaxies is presented. We adopt
three numbers from observations of the Milky Way disc, the local surface mass
density, the stellar scale length (of the assumedly exponential disc) and the
amplitude of the (assumedly flat) rotation curve, and physically, the (local)
dynamical Kennicutt star formation prescription, standard chemical evolution
equations assuming and a model for spectral evolution of stellar populations.
We can determine the detailed evolution of the model with only the addition of
standard cosmological scalings with time of the dimensional parameters. A
surprising wealth of detailed specifications follows from this prescription
including the gaseous infall rate as a function of radius and time, the
distribution of stellar ages and metallicities with time and radius, surface
brightness profiles at different wavelengths, colours etc. At the solar
neighbourhood stars start to form ago at an increasing rate
peaking 4 billion years ago and then slowly declining in good agreement with
observations. The mean age of long lived stars at the solar neighbourhood is
about . The local surface density of the stars and gas are 35 and , respectively. The metallicity distribution of the stars at
the solar radius is narrow with a peak at .Both a
Salpeter IMF and a Chabrier IMF are consistent with observations. Comparisons
with the current and local fossil evidence provides support for the model which
can then be used to assess other local disc galaxies, the evolution of disc
galaxies in deep optical surveys and also for theoretical investigations such
as simulations of merging disc galaxies (abbreviated).Comment: acceppted for publication in MNRA
Interstellar Deuterium, Nitrogen, and Oxygen Abundances Toward BD +28 4211: Results from the Far Ultraviolet Spectroscopic Explorer
High resolution far-ultraviolet spectra of the O-type subdwarf BD +28 4211
were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure
the interstellar deuterium, nitrogen, and oxygen abundances in this direction.
The interstellar D I transitions are analyzed down to Lyman iota at 920.7 A.
The star was observed several times at different target offsets in the
direction of spectral dispersion. The aligned and coadded spectra have high
signal-to-noise ratios (S/N =50-100). D I, N I, and O I transitions were
analyzed with curve-of-growth and profile fitting techniques. A model of
interstellar molecular hydrogen on the line of sight was derived from H2 lines
in the FUSE spectra and used to help analyze some features where blending with
H2 was significant. The H I column density was determined from high resolution
HST/STIS spectra of Lyman alpha to be log(N HI) = 19.846+/-0.035 (2 sigma),
which is higher than is typical for sight lines in the local ISM studied for
D/H. We found that D/H =(1.39+/-0.21) E-5 (2 sigma) and O/H = (2.37+/-0.55) E-4
(2 sigma). O/H toward BD +28 4211 appears to be significantly below the mean
O/H ratio for the ISM and the Local Bubble.Comment: 33 pages, 12 figures, accepted for publication in The Astrophysical
Journal Supplemen
The habitability of Proxima Centauri b I. Irradiation, rotation and volatile inventory from formation to the present
International audienceProxima b is a planet with a minimum mass of 1.3 MEarth orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the Sun's closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time evolution of the star's spectrum, which is essential for modeling the flux received over Proxima b's lifetime. We also show that Proxima b's obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet's eccentricity and level of triaxiality. Next we consider the evolution of Proxima b's water inventory. We use our spectral energy distribution to compute the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find that Proxima b is likely to have lost less than an Earth ocean's worth of hydrogen before it reached the HZ 100-200 Myr after its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We conclude that Proxima b is a viable candidate habitable planet
A chemical survey of exoplanets with ARIEL
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio
Rosseland and Planck mean opacities for primordial matter
We present newly calculated low-temperature opacities for gas with a
primordial chemical composition. In contrast to earlier calculations which took
a pure metal-free Hydrogen/Helium mixture, we take into account the small
fractions of Deuterium and Lithium as resulting from Standard Big Bang
Nucleosynthesis. Our opacity tables cover the density range -16 < log rho [g
cm^{-3}] < -2 and temperature range of 1.8 < T [K] < 4.6, while previous tables
were usually restricted to T > 10^3 K. We find that, while the presence of
Deuterium does not significantly alter the opacity values, the presence of
Lithium gives rise to major modifications of the opacities, at some points
increasing it by approximately 2 orders of magnitude relative to pure
Hydrogen/Helium opacities.Comment: 16 pages, 8 figures, submitted to MNRAS, all figures in grey-scale
and at reduced resolution, for high-res colour PDF see
http://www.ita.uni-heidelberg.de/~mm/publications/MayerDuschl-2.pd
Future exoplanet research: XUV (EUV and X-ray) detection and characterization
This chapter gives an overview of the current status of XUV research in exoplanets and highlights the prospects of future observations. Fundamental questions about the formation and the physical and chemical evolution of exoplanets, particularly hot Jupiters, are addressed through the different lines of XUV research: these comprise XUV irradiation of planetary atmospheres by the host stars, and consequent mass loss and atmospheric evaporation; X-ray and UV transits in exoplanet systems; and Star-Planet Interactions, most often determined by magnetic and tidal forces. While no other UV instrumentation as powerful as that carried by the Hubble Space Telescope will be available for detailed studies in the foreseeable future, the discovery potential of future revolutionary X-ray observatories, such as ATHENA and Lynx, will provide accurate atmosphere characterization and will make strides towards establishing the physics of the interactions between exoplanets and their host stars
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