From exoplanets to exocomets Ferlet, R
Journal of physics. Conference series,
07/2019, Volume:
1269, Issue:
1
Journal Article
Peer reviewed
Open access
To date (June 2017), more than 3610 planets orbiting other stars than our Sun are known. We shall briefly review the main detection methods, together with some of the big surprises which arose since ...the discovery of the first exoplanet around a solar type star in 1995. It is now also possible to characterize the atmospheres of few extrasolar planets and exocomets become detectable.
Atmospheric 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.
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 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%.
Aims. The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called "machos") could be a major component of the dark matter halo of the Milky Way galaxy. To ...this end, EROS- 2 monitored over 6.7 years 33\times10 similar to stars in the Magellanic clouds for microlensing events caused by such objects. Methods. In this work, we use only a subsample of 7\times10 similar to bright stars spread over 84\,\rm deg arrow up of the LMC and 9\,\rm deg arrow up of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent. Results. Using this sample of bright stars, only one candidate event was found, whereas similar to 39 events would have been expected if the Halo were entirely populated by objects of mass M\sim0.4 similar to M_{\odot}. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud ( LMC) due to such lenses is \tau<0.36\times10 (95% CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range 0.6\times10 contains as a subset _\odot<M<15 similar to M_{\odot} are ruled out as the primary occupants of the Milky Way Halo.
The young planetary system surrounding the star β Pictoris harbours active minor bodies. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as ...happened early in the history of our Solar System. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.
Observations of transits of the hot giant exoplanet HD 189733b in the unresolved H i Lyman-α line show signs of hydrogen escaping the upper atmosphere of the planet. New resolved Lyman-α observations ...obtained with the STIS spectrograph onboard the Hubble Space Telescope in April 2010 and September 2011 confirmed that the planet is evaporating, and furthermore discovered significant temporal variations in the physical conditions of its evaporating atmosphere. Here we present a detailed analysis of the September 2011 observations of HD 189733b, when an atmospheric signature was detected. We present specific methods to find and characterize this absorption signature of escaping hydrogen in the Lyman-α line, and to calculate its false-positive probability, found to be 3.6%. Taking advantage of the spectral resolution and high sensitivity of the STIS spectrograph, we also present new results on temporal and spectro-temporal variability of this absorption feature. We also report the observation of HD 189733b in other lines (Si iii at 1206.5 Å, N v at 1240 Å). Variations in these lines could be explained either by early occultation by a bow-shock rich in highly ionized species, or by stellar variations.
Four transits of the planet orbiting the star HD 209458 were observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The wavelength domain (1180-1710 Å) includes H ...I as well as C I, C II, C IV, N V, O I, S I, Si II, Si III, and Si IV lines. During the transits, absorptions are detected in H I, O I, and C II (5%+/-2%, 13%+/-4.5%, and 7.5%+/-3.5%, respectively). No absorptions are detected for other lines. The 5% mean absorption over the whole H I Lyα line is consistent with the previous detection completed in 2003 at higher resolution (Vidal-Madjar et al.). The absorption depths in O I and C II show that oxygen and carbon are present in the extended upper atmosphere of HD 209458b (nicknamed ``Osiris''). These species must be carried out up to the Roche lobe and beyond, most likely in a state of hydrodynamic escape.
Aims. The presence of titanium oxide (TiO) and vanadium oxide (VO) gas phase species is searched for in the atmosphere of the hot Jupiter HD 209458b. Methods. We compared a model for the planet's ...transmitted spectrum to multi-wavelength eclipse-depth measurements (from 3000 to 10 000 Å) using archived HST-STIS time series spectra. We make use of these observations to search for spectral signatures from extra absorbers in the planet atmosphere between 6000 and 8000 Å. Results. Along with sodium depletion and Rayleigh scattering recently published for this exoplanet atmosphere, an extra absorber of uncertain origin, redward of the sodium lines, is present in the atmosphere of the planet. Furthermore, this planet has a stratosphere experiencing a thermal inversion caused by the capture of optical stellar flux by absorbers at altitude. Recent models have predicted that the presence of TiO and VO in the atmosphere of HD 209458b may be responsible for this temperature inversion. Although no specific TiO and VO spectral band head signatures have been identified unambiguously in the observed spectrum, we suggest here that the opacities of those molecules are possible candidates to explain the remaining continuous broad band absorption observed between 6200 and 8000 Å. To match the data reasonably well, the abundances of TiO and VO molecules are evaluated from ten to one thousand times below solar. This upper limit result is in agreement with expected variations with altitude due to depletion effects such as condensation.
The young and nearby star β Pictoris (β Pic) is surrounded by a debris disk composed of dust and gas known to host a myriad evaporating exocomets, planetesimals and at least one planet. At an edge-on ...inclination, as seen from Earth, this system is ideal for debris disk studies providing an excellent opportunity to use absorption spectroscopy to study the planet forming environment. Using the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST) we observe the most abundant element in the disk, hydrogen, through the H I Lyman α (Ly-α) line. We present a new technique to decrease the contamination of the Ly-α line by geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique allows us to shift the Ly-α line of the astrophysical target away from the contaminating airglow emission revealing more of the astrophysical line profile. This new AVM technique, together with subtraction of an airglow emission map, allows us to analyse the shape of the β Pic Ly-α emission line profile and from it, calculate the column density of neutral hydrogen surrounding β Pic. The column density of hydrogen in the β Pic stable gas disk at the stellar radial velocity is measured to be log (NH/ 1 cm2) ≪ 18.5. The Ly-α emission line profile is found to be asymmetric and we propose that this is caused by H I falling in towards the star with a bulk radial velocity of 41 ± 6 km s-1 relative to β Pic and a column density of log (NH/ 1 cm2) = 18.6 ± 0.1. The high column density of hydrogen relative to the hydrogen content of CI chondrite meteorites indicates that the bulk of the hydrogen gas does not come from the dust in the disk. This column density reveals a hydrogen abundance much lower than solar, which excludes the possibility that the detected hydrogen could be a remnant of the protoplanetary disk or gas expelled by the star. We hypothesise that the hydrogen gas observed falling towards the star arises from the dissociation of water originating from evaporating exocomets.
Context. Atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging ...Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process and to determine the evaporation state of other “hot Jupiters”. Aims. Here, we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Methods. Eight HST orbits were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail. Results. Transit depths of $(9.6\pm7.0)\%$ and $(5.3\pm10.0)\%$ were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of $(8.0\pm5.7)\%$, in good agreement with previous studies. Conclusions. The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate.
PDF
