165 research outputs found
Metal-rich Damped/sub-Damped Lyman-alpha Quasar Absorbers at z<1
Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background
quasar, provide the most detailed probes available of element abundances in the
Universe over > 90 % of its age. DLAs can be used to observationally measure
the global mean metallicity in the Universe and its evolution with time.
Paradoxically, these observations are more difficult at lower redshifts, where
the absorber rest-frame UV spectra are cut-off due to the atmospheric
absorption. We present here high-resolution VLT/UVES observations of several
elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9. We detect
Mg I, Mg II, Fe II, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more
than double the high-resolution sample of [Zn/H] at z<1. We also report the
discovery of three metal-rich systems, whereas most previous measurements show
low N(HI)-weighted mean metallicity projecting to about 1/6th solar level at
z=0. We derive [Zn/H]=-0.11+/-0.04 at z_abs=0.725, [Zn/H]=-0.54+/-0.20 at
z_abs=0.740 and [Zn/H]=-0.49+/-0.22 at z_abs=0.652, plus one additional upper
limit ([Zn/H]<-0.36 at z_abs=0.842). These measurements confirm the existence
of quasar absorbers with relatively high metallicities based on abundance
estimates free from the effect of dust depletion. Possible implications of
these results for the metallicity of neutral gas phase in the past ~ 8 Gyr are
presented and compared with models.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
FUSE observations of the HI interstellar gas of IZw18
We present the analysis of FUSE observations of the metal-deficient dwarf
galaxy IZw18. We measured column densities of HI, NI, OI, ArI, SiII, and FeII.
The OI/HI ratio (log(OI/HI)=-4.7^{+0.8}_{-0.6}) is consistent with the O/H
ratio observed in the HII regions (all uncertainties are 2-sigma). If the
oxygen is depleted in the HI region compared to the HII regions, the depletion
is at most 0.5dex. This is also consistent with the log(O/H) ratios ~-5
measured with FUSE in the HI regions of other blue compact dwarf galaxies. With
log(NI/OI)=-2.4^{+0.6}_{-0.8}, the measured NI/OI ratio is lower than expected
for primary nitrogen. The determination of the NII column density is needed to
discriminate between a large ionization of NI or a possible nitrogen
deficiency. The neutral argon is also apparently underabundant, indicating that
ionization into ArII is likely important. The column densities of the other
alpha-chain elements SiII and ArI favor the lower edge of the permitted range
of OI column density, log(N(OI))~16.3.Comment: Accepted for publication in A&
A High Deuterium Abundance at z=0.7
Of the light elements, the primordial abundance of deuterium, (D/H)_p,
provides the most sensitive diagnostic for the cosmological mass density
parameter Omega_B. Recent high redshift (D/H) measurements are highly
discrepant, although this may reflect observational uncertainties. The larger
(D/H) values, which imply a low Omega_B and require the Universe to be
dominated by non-baryonic matter (dynamical studies indicate a higher total
density parameter), cause problems for galactic chemical evolution models since
they have difficulty in reproducing the large decline down to the lower
present-day (D/H). Conversely, low (D/H) values imply an Omega_B greater than
derived from ^7Li and ^4He abundance measurements, and may require a deuterium
abundance evolution that is too low to easily explain. Here we report the first
measurement at intermediate redshift, where the observational difficulties are
smaller, of a gas cloud with ideal characteristics for this experiment. Our
analysis of the z = 0.7010 absorber toward 1718+4807 indicates (D/H) = 2.0 +/-
0.5 x 10^{-4} which is in the high range. This and other independent
observations suggests there may be a cosmological inhomogeneity in (D/H)_p of
at least a factor of ten.Comment: 6 pages, 1 figur
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Copyright © ESO, 2012Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-α line. Here we present spectrally resolved Lyman-α transit observations of the exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4 ± 3.6% between velocities of −230 to −140 km s-1. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 109 g s-1, a stellar wind with a velocity of 190 km s-1 and a temperature of ~105 K. An X-ray flare from the active star seen with Swift/XRT 8 h before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet’s atmosphere and stellar variations.STScIFondation Simone et Cino Del Duc
The high-energy environment in the super-earth system CoRoT-7
High-energy irradiation of exoplanets has been identified to be a key
influence on the stability of these planets' atmospheres. So far,
irradiation-driven mass-loss has been observed only in two Hot Jupiters, and
the observational data remain even more sparse in the super-earth regime. We
present an investigation of the high-energy emission in the CoRoT-7 system,
which hosts the first known transiting super-earth. To characterize the
high-energy XUV radiation field into which the rocky planets CoRoT-7b and
CoRoT-7c are immersed, we analyzed a 25 ks XMM-Newton observation of the host
star. Our analysis yields the first clear (3.5 sigma) X-ray detection of
CoRoT-7. We determine a coronal temperature of ca. 3 MK and an X-ray luminosity
of 3*10^28 erg/s. The level of XUV irradiation on CoRoT-7b amounts to ca. 37000
erg/cm^2/s. Current theories for planetary evaporation can only provide an
order-of-magnitude estimate for the planetary mass loss; assuming that CoRoT-7b
has formed as a rocky planet, we estimate that CoRoT-7b evaporates at a rate of
about 1.3*10^11 g/s and has lost ca. 4-10 earth masses in total.Comment: 5 pages, accepted for publication by Astronomy & Astrophysic
Magnesium in the atmosphere of the planet HD 209458 b: Observations of the thermosphere-exosphere transition region
Copyright © ESO, 2013The planet HD 209458 b is one of the most well studied hot-Jupiter exoplanets. The upper atmosphere of this planet has been observed through ultraviolet/optical transit observations with H i observation of the exosphere revealing atmospheric escape. At lower altitudes just below the thermosphere, detailed observations of the Na i absorption line has revealed an atmospheric thermal inversion. This thermal structure is rising toward high temperatures at high altitudes, as predicted by models of the thermosphere, and could reach ~ 10 000 K at the exobase level. Here, we report new near ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations of atmospheric absorptions during the planetary transit of HD 209458 b. We report absorption in atomic magnesium (Mg i), while no signal has been detected in the lines of singly ionized magnesium (Mg ii). We measure the Mg i atmospheric absorption to be 6.2 ± 2.9% in the velocity range from − 62 to − 19 km s-1. The detection of atomic magnesium in the planetary upper atmosphere at a distance of several planetary radii gives a first view into the transition region between the thermosphere and the exobase, where atmospheric escape takes place. We estimate the electronic densities needed to compensate for the photo-ionization by dielectronic recombination of Mg+ to be in the range of 108−109 cm-3. Our finding is in excellent agreement with model predictions at altitudes of several planetary radii. We observe Mg i atoms escaping the planet, with a maximum radial velocity (in the stellar rest frame) of −60 km s-1. Because magnesium is much heavier than hydrogen, the escape of this species confirms previous studies that the planet’s atmosphere is undergoing hydrodynamic escape. We compare our observations to a numerical model that takes the stellar radiation pressure on the Mg i atoms into account. We find that the Mg i atoms must be present at up to ~7.5 planetari radii altitude and estimate an Mg i escape rate of ~3 × 107 g s-1. Compared to previous evaluations of the escape rate of H i atoms, this evaluation is compatible with a magnesium abundance roughly solar. A hint of absorption, detected at low level of significance, during the post-transit observations, could be interpreted as a Mg i cometary-like tail. If true, the estimate of the absorption by Mg i would be increased to a higher value of about 8.8 ± 2.1%.Centre National d’Études Spatiales (CNES)French Agence Nationale de la Recherche (ANR)NASA Exoplanet Science Institute (NExScI) - Sagan Exoplanet Fellowship programSTFC (Science & Technology Facilities Council)European Commissions Seventh Framework Programme - Marie Curie Intra-European Fellowshi
Magnesium in the atmosphere of the planet HD 209458 b: observations of the thermosphere-exosphere transition region
The planet HD 209458 b is one of the most well studied hot-Jupiter exoplanets. The upper atmosphere of this planet has been observed through ultraviolet/optical transit observations with H I observation of the exosphere revealing atmospheric escape. At lower altitudes just below the thermosphere, detailed observations of the Na I absorption line has revealed an atmospheric thermal inversion. This thermal structure is rising toward high temperatures at high altitudes, as predicted by models of the thermosphere, and could reach ~ 10 000 K at the exobase level. Here, we report new near ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations of atmospheric absorptions during the planetary transit of HD 209458 b. We report absorption in atomic magnesium (Mg I), while no signal has been detected in the lines of singly ionized magnesium (Mg II). We measure the Mg I atmospheric absorption to be 6.2±2.9% in the velocity range from −62 to −19 km s^(-1). The detection of atomic magnesium in the planetary upper atmosphere at a distance of several planetary radii gives a first view into the transition region between the thermosphere and the exobase, where atmospheric escape takes place. We estimate the electronic densities needed to compensate for the photo-ionization by dielectronic recombination of Mg+ to be in the range of 10^8−10^9 cm^(-3). Our finding is in excellent agreement with model predictions at altitudes of several planetary radii. We observe Mg I atoms escaping the planet, with a maximum radial velocity (in the stellar rest frame) of −60 km s^(-1). Because magnesium is much heavier than hydrogen, the escape of this species confirms previous studies that the planet’s atmosphere is undergoing hydrodynamic escape. We compare our observations to a numerical model that takes the stellar radiation pressure on the Mg I atoms into account. We find that the Mg I atoms must be present at up to ~7.5 planetari radii altitude and estimate an Mg I escape rate of ~3 × 10^7 gs^(-1). Compared to previous evaluations of the escape rate of H I atoms, this evaluation is compatible with a magnesium abundance roughly solar. A hint of absorption, detected at low level of significance, during the post-transit observations, could be interpreted as a Mg I cometary-like tail. If true, the estimate of the absorption by Mg I would be increased to a higher value of about 8.8 ± 2.1%
HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
We have detected new HD absorption systems at high redshifts, z_abs=2.626 and
z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170,
respectively. Each of these systems consists of two subsystems. The HD column
densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2)
and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208
and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20
for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for
three of these subsystems has been found to be considerably higher than its
values typical of clouds in our Galaxy. We discuss the problem of determining
the primordial deuterium abundance, which is most sensitive to the baryon
density of the Universe \Omega_{b}. Using a well-known model for the chemistry
of a molecular cloud, we have estimated the isotopic ratio
D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density
\Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with
\Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic
microwave background radiation anisotropy. However, in high-redshift clouds,
under conditions of low metallicity and low dust content, hydrogen may be
incompletely molecularized even in the case of self-shielding. In this
situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic
ratio. We have estimated the cloud molecularization dynamics and the influence
of cosmological evolutionary effects on it
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Atmospheric escape has been detected from the exoplanet HD 209458b through
transit observations of the hydrogen Lyman-alpha line. Here we present
spectrally resolved Lyman-alpha transit observations of the exoplanet HD
189733b at two different epochs. These HST/STIS observations show for the first
time, that there are significant temporal variations in the physical conditions
of an evaporating planetary atmosphere. While atmospheric hydrogen is not
detected in the first epoch observations, it is observed at the second epoch,
producing a transit absorption depth of 14.4+/-3.6% between velocities of -230
to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from
radiation pressure alone and require an additional acceleration mechanism, such
as interactions with stellar wind protons. The observed absorption can be
explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9
g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the
second-epoch observation supports the idea that the observed changes within the
upper atmosphere of the planet can be caused by variations in the stellar wind
properties, or by variations in the stellar energy input to the planetary
escaping gas (or a mix of the two effects). These observations provide the
first indication of interaction between the exoplanet's atmosphere and stellar
variations.Comment: To be published in A&A Letters, June 28, 201
Can gas in young debris disks be constrained by their radial brightness profiles?
Disks around young stars are known to evolve from optically thick,
gas-dominated protoplanetary disks to optically thin, almost gas-free debris
disks. It is thought that the primordial gas is largely removed at ages of ~10
Myr, but it is difficult to discern the true gas densities from gas
observations. This suggests using observations of dust: it has been argued that
gas, if present with higher densities, would lead to flatter radial profiles of
the dust density and surface brightness than those actually observed. However,
here we show that these profiles are surprisingly insensitive to variation of
the parameters of a central star, location of the dust-producing planetesimal
belt, dustiness of the disk and - most importantly - the parameters of the
ambient gas. This result holds for a wide range of gas densities (three orders
of magnitude), for different radial distributions of the gas temperature, and
different gas compositions. The brightness profile slopes of -3...-4 we find
are the same that were theoretically found for gas-free debris disks, and they
are the same as actually retrieved from observations of many debris disks. Our
specific results for three young (10-30 Myr old), spatially resolved, edge-on
debris disks (beta Pic, HD 32297, and AU Mic) show that the observed radial
profiles of the surface brightness do not pose any stringent constraints on the
gas component of the disk. We cannot exclude that outer parts of the systems
may have retained substantial amounts of primordial gas which is not evident in
the gas observations (e.g. as much as 50 Earth masses for beta Pic). However,
the possibility that gas, most likely secondary, is only present in little to
moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in
the beta Pic disk), remains open, too.Comment: Accepted for publication in Astronomy and Astrophysic
- …