Hydrogen-dominated Atmospheres Research Articles

XUV-Exposed, Non-Hydrostatic Hydrogen-Rich Upper Atmospheres of Terrestrial Planets. Part II: Hydrogen Coronae and Ion Escape

We studied the interactions between the stellar wind plasma flow of a typical M star, such as GJ 436, and the hydrogen-rich upper atmosphere of an Earth-like planet and a "super-Earth" with a radius of 2 R(Earth) and a mass of 10 M(Earth), located within the habitable zone at ∼0.24 AU. We investigated the formation of extended atomic hydrogen coronae under the influences of the stellar XUV flux (soft X-rays and EUV), stellar wind density and velocity, shape of a planetary obstacle (e.g., magnetosphere, ionopause), and the loss of planetary pickup ions on the evolution of hydrogen-dominated upper atmospheres. Stellar XUV fluxes that are 1, 10, 50, and 100 times higher compared to that of the present-day Sun were considered, and the formation of high-energy neutral hydrogen clouds around the planets due to the charge-exchange reaction under various stellar conditions was modeled. Charge-exchange between stellar wind protons with planetary hydrogen atoms, and photoionization, lead to the production of initially cold ions of planetary origin. We found that the ion production rates for the studied planets can vary over a wide range, from ∼1.0×10²⁵ s⁻¹ to ∼5.3×10³⁰ s⁻¹, depending on the stellar wind conditions and the assumed XUV exposure of the upper atmosphere. Our findings indicate that most likely the majority of these planetary ions are picked up by the stellar wind and lost from the planet. Finally, we estimated the long-time nonthermal ion pickup escape for the studied planets and compared them with the thermal escape. According to our estimates, nonthermal escape of picked-up ionized hydrogen atoms over a planet's lifetime within the habitable zone of an M dwarf varies between ∼0.4 Earth ocean equivalent amounts of hydrogen (EO(H)) to <3 EO(H) and usually is several times smaller in comparison to the thermal atmospheric escape rates.

A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members

We report the discovery of two new pulsating extremely low-mass (ELM) white dwarfs (WDs), SDSS J161431.28+191219.4 (hereafter J1614) and SDSS J222859.93+362359.6 (hereafter J2228). Both WDs have masses <0.25 Msun and thus likely harbor helium cores. Spectral fits indicate these are the two coolest pulsating WDs ever found. J1614 has Teff = 8880 +/- 170 K and log g = 6.66 +/- 0.14, which corresponds to a roughly 0.19 Msun WD. J2228 is considerably cooler, with a Teff = 7870 +/- 120 K and log g = 6.03 +/- 0.08, which corresponds to a roughly 0.16 Msun WD, making it the coolest and lowest-mass pulsating WD known. There are multiple ELM WDs with effective temperatures between the warmest and coolest known ELM pulsators that do not pulsate to observable amplitudes, which questions the purity of the instability strip for low-mass WDs. In contrast to the CO-core ZZ Ceti stars, which are believed to represent a stage in the evolution of all such WDs, ELM WDs may not all evolve as a simple cooling sequence through an instability strip. Both stars exhibit long-period variability (1184-6235 s) consistent with non-radial g-mode pulsations. Although ELM WDs are preferentially found in close binary systems, both J1614 and J2228 do not exhibit significant radial-velocity variability, and are perhaps in low-inclination systems or have low-mass companions. These are the fourth and fifth pulsating ELM WDs known, all of which have hydrogen-dominated atmospheres, establishing these objects as a new class of pulsating WD.

A FRAMEWORK FOR CHARACTERIZING THE ATMOSPHERES OF LOW-MASS LOW-DENSITY TRANSITING PLANETS

We perform modeling investigations to aid in understanding the atmospheres and composition of small planets of ~2-4 Earth radii, which are now known to be common in our galaxy. GJ 1214b is a well studied example whose atmospheric transmission spectrum has been observed by many investigators. Here we take a step back from GJ 1214b to investigate the role that planetary mass, composition, and temperature play in impacting the transmission spectra of these low-mass low-density (LMLD) planets. Under the assumption that these planets accrete modest hydrogen-dominated atmospheres and planetesimals, we use population synthesis models to show that predicted metal enrichments of the H/He envelope are high, with metal mass fraction Z_env values commonly 0.6 to 0.9, or ~100 to 400+ times solar. The high mean molecular weight of such atmospheres (\mu ~ 5-12) would naturally help to flatten the transmission spectrum of most LMLD planets. The high metal abundance would also provide significant condensible material for cloud formation. It is known that the H/He abundance in Uranus and Neptune decreases with depth, and we show that atmospheric evaporation of LMLD planets could expose atmospheric layers with gradually higher Z_env. However, values of Z_env close to solar composition can also arise, so diversity should be expected. Photochemically produced hazes, potentially due to methane photolysis, are another possibility for obscuring transmission spectra. Such hazes may not form above T_eq of ~800-1100 K, which is testable if such warm, otherwise low mean molecular weight atmospheres are stable against atmospheric evaporation. We find that available transmission data are consistent with relatively high mean molecular weight atmospheres for GJ 1214b and "warm Neptune" GJ 436b. We examine future prospects for characterizing GJ 1214b with Hubble and the James Webb Space Telescope.

Probing the blow-off criteria of hydrogen-rich ‘super-Earths’

The discovery of transiting "super-Earths" with inflated radii and known masses such as Kepler-11b-f, GJ 1214b and 55 Cnc e, indicates that these exoplanets did not lose their nebula-captured hydrogen-rich, degassed or impact-delivered protoatmospheres by atmospheric escape processes. Because hydrodynamic blow-off of atmospheric hydrogen atoms is the most efficient atmospheric escape process we apply a time-dependent numerical algorithm which is able to solve the system of 1-D fluid equations for mass, momentum, and energy conservation to investigate the criteria under which "super-Earths" with hydrogen-dominated upper atmospheres can experience hydrodynamic expansion by heating of the stellar XUV (soft X-rays and extreme ultraviolet) radiation and thermal escape via blow-off. Depending on orbit location, XUV flux, heating efficiency and the planet's mean density our results indicate that the upper atmospheres of all "super-Earths" can expand to large distances, so that besides of Kepler-11c all of them experience atmospheric mass-loss due to Roche lobe overflow. The atmospheric mass-loss of the studied "super-Earths" is one to two orders of magnitude lower compared to that of "hot Jupiters" such as HD 209458b, so that one can expect that these exoplanets cannot lose their hydrogen-envelopes during their remaining lifetimes.

C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES

Until recently, infrared observations of exoplanetary atmospheres have typically been interpreted using models that assumed solar elemental abundances. With the chemical composition fixed, attempts have been made to classify hot Jupiter atmospheres on the basis of stellar irradiation. However, recent observations have revealed deviations from predictions based on such classification schemes, and chemical compositions retrieved from some datasets have also indicated non-solar abundances. In hot hydrogen-dominated atmospheres, the C/O ratio critically influences the relative concentrations of several spectroscopically dominant molecules. Between a C/O of 0.5 (solar value) and 2, the H2O and CH4 abundances can vary by several orders of magnitude in the observable atmosphere, and new hydrocarbon species such as HCN and C2H2 become prominent for C/O >= 1, while the CO abundance remains almost unchanged. Furthermore, a C/O >= 1 can preclude a strong thermal inversion due to TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturally under-abundant for C/O >= 1. We, therefore, suggest a new two-dimensional classification scheme for hydrogen-dominated exoplanetary atmospheres with irradiation (or temperature) and C/O ratio as the two dimensions. We define four classes in this 2-D space (O1, O2, C1, and C2) with distinct chemical, thermal and spectral properties. We characterize the thermal structures and C/O ratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, and WASP-12b) in the framework of our proposed 2D classification scheme. While the data for several systems in our sample are consistent with carbon-rich atmospheres (i.e. C/O >= 1), new observations are required to conclusively constrain the C/O ratios in their atmospheres. We discuss how observations using existing and forthcoming facilities can constrain C/O ratios in exoplanetary atmospheres.

ON THE STABILITY OF SUPER-EARTH ATMOSPHERES

We investigate the stability of super Earth atmospheres around M stars using a 7-parameter, analytical framework. We construct stability diagrams in the parameter space of exoplanetary radius versus semi-major axis and elucidate the regions in which the atmospheres are stable against the condensation of their major constituents, out of the gas phase, on their permanent nightside hemispheres. We find that super Earth atmospheres which are nitrogen-dominated ("Earth-like") occupy a smaller region of allowed parameter space, compared to hydrogen-dominated atmospheres, because of the dual effects of diminished advection and enhanced radiative cooling. Furthermore, some super Earths which reside within the habitable zones of M stars may not possess stable atmospheres, depending on the mean molecular weight and infrared photospheric pressure of their atmospheres. We apply our stability diagrams to GJ 436b and GJ 1214b, and demonstrate that atmospheric compositions with high mean molecular weights are disfavoured if these exoplanets possess solid surfaces and shallow atmospheres. Finally, we construct stability diagrams tailored to the Kepler dataset, for G and K stars, and predict that about half of the exoplanet candidates are expected to habour stable atmospheres if Earth-like conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability diagram for G stars.

THE ATMOSPHERIC CHEMISTRY OF GJ 1214b: PHOTOCHEMISTRY AND CLOUDS

Recent observations of the transiting super-Earth GJ 1214b reveal that its atmosphere may be hydrogen-rich or water-rich in nature, with clouds or hazes potentially affecting its transmission spectrum in the optical and very-near-IR. Here we further examine the possibility that GJ 1214b does indeed possess a hydrogen-dominated atmosphere, which is the hypothesis that is favored by models of the bulk composition of the planet. We study the effects of non-equilibrium chemistry (photochemistry, thermal chemistry, and mixing) on the planet's transmission spectrum. We furthermore examine the possibility that clouds could play a significant role in attenuating GJ 1214b's transmission spectrum at short wavelengths. We find that non-equilibrium chemistry can have a large effect on the overall chemical composition of GJ 1214b's atmosphere, however these changes mostly take place above the height in the atmosphere that is probed by transmission spectroscopy. The effects of non-equilibrium chemistry on GJ 1214b's transmission spectrum are therefore minimal, with the largest effects taking place if the planet's atmosphere has super-solar metallicity and a low rate of vertical mixing. Interestingly, we find that the best fit to the observations of GJ 1214b's atmosphere in transmission occur if the planet's atmosphere is deficient in CH4, and possesses a cloud layer at a pressure of ~200 mbar. This is consistent with a picture of efficient methane photolysis, accompanied by formation of organic haze that obscures the lower atmosphere of GJ 1214b at optical wavelengths. However, for methane to be absent from GJ 1214b's transmission spectrum, UV photolysis of this molecule must be efficient at pressures of greater than ~1 mbar, whereas we find that methane only photolyzes to pressures less than 0.1 mbar, even under the most optimistic assumptions. (Abridged)

OBSERVATIONAL EVIDENCE FOR A METAL-RICH ATMOSPHERE ON THE SUPER-EARTH GJ1214b

We report observations of two consecutive transits of the warm super-Earth exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera instrument on-board the Spitzer Space Telescope. The two transit light curves allow for the determination of the transit parameters for this system. We find these paremeters to be consistent with the previously determined values and no evidence for transit timing variations. The main investigation consists of measuring the transit depths in each bandpass to constrain the planet's transmission spectrum. Fixing the system scale and impact parameters, we measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6 and 4.5 microns, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical yields constraints on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free, solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma confidence. This independently confirms a recent finding that was based on a measurement of the planet's transmission spectrum using the VLT. The Spitzer, MEarth, and VLT observations together yield a remarkably flat transmission spectrum over the large wavelength domain spanned by the data. Consequently, cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H2O by volume to be consistent with all the observations.

VISITORS FROM THE HALO: 11 Gyr OLD WHITE DWARFS IN THE SOLAR NEIGHBORHOOD

We report the discovery of three nearby old halo white dwarf candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 square degree proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3m telescopes, and they display proper motions of 0.4-0.5 arcsec/yr. Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere white dwarfs with Teff = 3700 - 4100 K. For average mass white dwarfs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km/s. Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and white dwarf cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field white dwarfs currently known. These three stars represent only a small fraction of the halo white dwarf candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo white dwarfs that can be used to constrain the age of the Galactic thick disk and halo.

The deep water abundance on Jupiter: New constraints from thermochemical kinetics and diffusion modeling

We have developed a one-dimensional thermochemical kinetics and diffusion model for Jupiter’s atmosphere that accurately describes the transition from the thermochemical regime in the deep troposphere (where chemical equilibrium is established) to the quenched regime in the upper troposphere (where chemical equilibrium is disrupted). The model is used to calculate chemical abundances of tropospheric constituents and to identify important chemical pathways for CO–CH 4 interconversion in hydrogen-dominated atmospheres. In particular, the observed mole fraction and chemical behavior of CO is used to indirectly constrain the jovian water inventory. Our model can reproduce the observed tropospheric CO abundance provided that the water mole fraction lies in the range (0.25–6.0) × 10 −3 in Jupiter’s deep troposphere, corresponding to an enrichment of 0.3–7.3 times the protosolar abundance (assumed to be H 2O/H 2 = 9.61 × 10 −4). Our results suggest that Jupiter’s oxygen enrichment is roughly similar to that for carbon, nitrogen, and other heavy elements, and we conclude that formation scenarios that require very large (>8× solar) enrichments in water can be ruled out. We also evaluate and refine the simple time-constant arguments currently used to predict the quenched CO abundance on Jupiter, other giant planets, and brown dwarfs.

White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy

The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than ~M_{sun} have been ruled out as major components of a Massive Astrophysical Compact Halo Object (MACHO) Galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. In this paper we evaluate the contribution to the dark matter content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For this purpose we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres. We also take into account detailed descriptions of the thick disk and the halo of our Galaxy as well as of a reliable model of the LMC. We find that the contribution of white dwarfs with hydrogen-deficient atmospheres moderately increases the theoretical estimate of the optical depth with respect to the value obtained when only hydrogen-rich white dwarfs are considered. We also find that the contribuiton of the thick disk population of white dwarfs is comparable to the halo contribution. However, the contributions of both the halo and the thick disk white-dwarf populations are still insufficient to explain the number of events observed by the MACHO team. Finally, we find that the contribution to the halo dark matter of the entire population under study is less than 10% at the 95% conficence level.

Seismological studies of ZZ Ceti stars - I. The model grid and the application to individual stars

We calculate and explore an extensive adiabatic model grid for pulsating white dwarfs with hydrogen-dominated atmospheres, the ZZ Ceti stars. We also compared the computed modes with the observed ones for five ZZ Ceti stars that are a representative sample of the whole class of pulsators. We describe our new approach for seismological studies, using the relative observed amplitudes to give weights for the periods in the fit and the external mass and temperature determinations as a guide. Our seismological study is clear evidence that seismology is indeed a powerful tool in the study of stellar structure and evolution.

Hydrogen Photoionization Cross Sections for Strong‐Field Magnetic White Dwarfs

A recently developed computational scheme, based on the complex-rotation method combined with a mixed Slater-Landau basis expansion, has been applied to calculations of photoionization from atomic hydrogen ground and excited states in a strong magnetic field typical of magnetic white dwarf stars. Photoionization cross sections are presented for the twelve states 1s0 ,2 s0 ,2 p0 ,2 p−1 ,3 s � , 3p0 ,3 p−1 ,3 d � ,3 d−1 ,3 d−2 ,4 f−2 ,a nd 4f−3 with magnetic field strengths from 23.5 to 2350 MG. In most cases, good agreement has been found between the current results and the limited theoretical data in the literature. New transitions and additional field strengths are given here which will be applicable to modeling the flux and polarization spectra of magnetic white dwarfs with hydrogen-dominated atmospheres.

Continuous absorption by He2(+) and H2(+) in cool white dwarfs

Photodissociation cross sections and absorption coefficents of the He2(+) molecular ions for transitions from the Chi (2)Epsilon(+) ground state to the vibrational continuum of the A (2)Epsilon(+) excited state have been determined for photon wavelengths lambda between 50 nm and 20 microns and for temperatures T between 4200 and 50,400 K and between 3150 and 25,200 K, respctivley. The cross sections are obtained through a fully quantum-mechanical method, and it is shown that inclusion of the high-lying rotational states is important for determination of the correct long-wavelengthh cross section amplitude and the peak wavelength. Free-free absorption coefficients are also obtained using the semiclassical method of Bates. The detection possibility of He2(+) and H2(+) in cool white dwarfs is dicussed and shown to be insignificant. An effectively temperature independent cutoff in the He2(+) photodissociation cross section in the EUV at approximately 60 nm appears to be the only possible observable feature. It is suggested that He2(+) may be responsible for an observed UV flux deficiency in some cool DB stars as well as in the DZ stars WD 0552 - 041 and WD 1334 + 039. We further suggest that the star WD 0752 - 676 currently assumed to be helium-rich may actually have a hydrogen-dominated atmosphere.

An Explanation for the Euv Spectrum of Feige 24

Feige 24 is a bright DA white dwarf which has been studied extensively both from ground-based and space-borne observatories. The best determination of its fundamental atmospheric parameters are that of Holberg, Wesemael, and Basile (1986) who have used detailed model atmosphere analyses in conjunction with optical, IUE, and Voyager data. They give log g = 7.23±0.35 and Te(103K) = 55±5. The question of the atmospheric composition is more involved as small traces of heavy elements would not be observable in the optical spectrum of such a hot, hydrogen-dominated atmosphere. If it were isolated, Feige 24 would presumably only show the usual bland optical spectrum of a typical DA white dwarf, i.e. the hydrogen Balmer line series, but the presence of a M dwarf companion complicates its spectrum. On the other hand, Feige 24 belongs to a handful of hot DA white dwarfs sufficiently bright that ultraviolet spectroscopy in the high resolution mode of the IUE has been possible. Following the theoretical expectation of Vauclair, Vauclair, and Greenstein (1979), it was discovered that the photosphere of Feige 24 contains small amounts of C, N, and Si (Dupree and Raymond 1982). Spectral synthesis techniques used by Wesemael, Henry, and Shipman (1984) indicate the following abundances: log(C/H)=−6.4±0.6, log(N/H)=−5.3±1.0, and log(Si/H)=−6.3±0.9. The most plausible explanation to account for these small abundances is the influence of selective radiative forces possibly coupled to a weak wind (Chayer et al. 1987).

COMPARISON OF THE BROAD-BAND AND MULTICHANNEL COLORS OF WHITE DWARFS

ABSTRACT The published braod-band UBV and multichannel spectrophotometry is correlated for the white dwarfs to provide the nonlinear color-color transformations between these systems. The hydrogen lines and continuum produce a bifurcation separating hydrogen-dominated and helium-dominated atmospheres in color-color diagrams, and account, in part, for the Eggen-Greenstein two sequences. The UBV color-luminosity diagram also is split over a wide range of temperatures; (U-V)0 is approximately calibrated by temperature for the two dominant composition types. Mapping of MV, (U-V)0 shows that most He-atmosphere in the steep dependence of MV on (U-V)0 from 8000 K to 13,000 K; (U-V)0 is a poor luminosity indicator in that range of effective temperature. Otherwise, good UBV photometry proves quite useful for the study of white dwarfs.

On the Maximum Exospheric Temperature of Hydrogen-Dominated Planetary Atmospheres

Abstract It is shown that under static conditions the maximum temperature attainable in the exospheres of hydrogen-dominated planetary atmospheres is of order 105 K when gravitational separation between hydrogen and other elements has occurred, and ∼104 K for an atmosphere of cosmic abundance.

On the Exospheric Temperature of Hydrogen-Dominated Planetary Atmospheres

Results of studies of hydrogen-dominated atmospheres in which exospheric temperatures were calculated to estimate the evaporative loss rates of the constituents. Expressions are given for the exospheric temperature of a two-component diffusive equilibrium model which consists of a lighter molecule dominating the thermosphere and absorbing solar EUV, and a heavier constituent providing radiative cooling in the vicinity of the mesopause. The model approximates well the assumed atmospheres, but the resulting expressions may have wider application. The dynamics of an expanding atmosphere are discussed. A three-dimensional outflow against the back pressure of the solar wind may be configured like a comet's tail.