We used the Wide-field Infrared Camera on the Canada-France-Hawaii telescope to observe four transits of the super-Earth planet GJ 1214b in the near-infrared. For each transit we observed in two bands nearly-simultaneously by rapidly switching the WIRCam filter wheel back and forth for the duration of the observations. By combining all our J-band (∼1.25 µm) observations we find a transit depth, analogous to the planet-to-star radius ratio squared, in this band of (R P J /R * ) 2 =1.338±0.013% -a value consistent with the optical transit depth reported by Charbonneau and collaborators. However, our best-fit combined Ks-band (∼2.15 µm) transit depth is deeper: (R P Ks /R * ) 2 =1.438±0.019%. Formally our Ks-band transits are deeper than the J-band transits observed simultaneously by a factor of (R P Ks /R P J ) 2 =1.072±0.018 -a 4σ discrepancy. The most straightforward explanation for our deeper Ks-band transit depth is a spectral absorption feature from the limb of the atmosphere of the planet; for the spectral absorption feature to be this prominent the atmosphere of GJ 1214b must have a large scale height and a low mean molecular weight. That is, its atmosphere would have to be hydrogen/helium dominated and this planet would be better described as a mini-Neptune. However, recently published observations from 0.78 -1.0 µm, by Bean and collaborators, show a lack of spectral features and transit depths consistent with those obtained by Charbonneau and collaborators. The most likely atmospheric composition for GJ 1214b that arises from combining all these observations is less clear; if the atmosphere of GJ 1214b is hydrogen/helium dominated then it must have either a haze layer that is obscuring transit depth differences at shorter wavelengths, or significantly different spectral features than current models predict. Our observations disfavour a water-world composition, but such a composition will remain a possibility for GJ 1214b, until observations reconfirm our deeper Ks-band transit depth or detect features at other wavelengths.
In this Paper, we have derived Cepheid period-luminosity (P-L) relations for the Large Magellanic Cloud (LMC) fundamental mode Cepheids, based on the data released from OGLE-III. We have applied an extinction map to correct for the extinction of these Cepheids. In addition to the V IW band P-L relations, we also include JHK and four Spitzer IRAC band P-L relations, derived by matching the OGLE-III Cepheids to the 2MASS and SAGE datasets, respectively. We also test the non-linearity of the Cepheid P-L relations based on extinction-corrected data. Our results (again) show that the LMC P-L relations are non-linear in V IJH bands and linear in KW and the four IRAC bands, respectively.
Context. A longstanding challenge for understanding classical Cepheids is the Cepheid mass discrepancy, where theoretical mass estimates using stellar evolution and stellar pulsation calculations have been found to differ by approximately 10−20%. Aims. We study the role of pulsation-driven mass loss during the Cepheid stage of evolution as a possible solution to this mass discrepancy. Methods. We computed stellar evolution models with a Cepheid mass-loss prescription and various amounts of convective core overshooting. The contribution of mass loss towards the mass discrepancy is determined using these models, Results. Pulsation-driven mass loss is found to trap Cepheid evolution on the instability strip, allowing them to lose about 5−10% of their total mass when moderate convective core overshooting, an amount consistent with observations of other stars, is included in the stellar models. Conclusions. We find that the combination of moderate convective core overshooting and pulsation-driven mass loss can solve the Cepheid mass discrepancy.
Betelgeuse, a nearby red supergiant, is a runaway star with a powerful stellar wind that drives a bow shock into its surroundings 1-4 . This picture has been challenged by the discovery of a dense and almost static shell 5 that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse's wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind 6 and forms an almost static, photoionization-confined shell. Other red supergiants should have significantly more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova lightcurve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.Red supergiants are massive stars near the end of their lives, and are direct progenitors of core-collapse supernovae 7, 8 . They evolve from O-and B-type stars (hot, luminous sources of ionizing photons), and so these stars are often found together, within or near star clusters 9 . As a result, the cool stellar winds of red supergiants are often photoionized by external radiation fields [10][11][12][13] . To calculate the radiation hydrodynamics of a photoionized red supergiant wind, we simplify the problem by assuming spherical symmetry. We use an approximate two-temperature equation of state for the gas, for which both the neutral and photoionized gases are isothermal with 1 temperatures T = T n and T i ≫ T n , respectively. The ionized and neutral isothermal sound speeds similarly satisfy a i ≫ a n . The photoionized part of the red supergiant wind is accelerated as a result of ionization heating 14 , whereas the neutral part is decelerated 6 if the wind speed through the ionization front, v n , satisfies v n ≤ 2a i .The resulting flow is depicted in Fig. 1. The outermost layer is the interface where the wind meets the interstellar medium. For static stars this is a spherical, detached shell, and for stars moving supersonically it is a bow shock. A photoionization-confined shell -a dense, shocked layer separating the neutral inner wind from the ionized outer wind -forms closer to the star. We identify this with the recently-discovered shell in Betelgeuse's circumstellar medium 5 .The properties of the photoionization-confined shell are calculated analytically and verified with simulations in Methods. Its outer boundary, R IF , is calculated following previous work 10 (Extended Data Fig. 1), and the standing shock radius, R shell , is obtained by requiring pressur...
An analytical derivation is presented for computing mass-loss rates of Cepheids by using the method of Castor, Abbott, & Klein (1975) modified to include a term for momentum input from pulsation and shocks generated in the atmosphere. Using this derivation, mass-loss rates of Cepheids are determined as a function of stellar parameters. When applied to a set of known Cepheids, the calculated mass-loss rates range from 10^{-10} to 10^{-7}M_{Sun}/yr, larger than if the winds were driven by radiation alone. Infrared excesses based on the predicted mass-loss rates are compared to observations from optical interferometry and IRAS, and predictions are made for Spitzer observations. The mass-loss rates are consistent with the observations, within the uncertainties of each. The rate of period change of Cepheids is discussed and shown to relate to mass loss, albeit the dependence is very weak. There is also a correlation between the large mass-loss rates and the Cepheids with slowest absolute rate of period change due to evolution through the instability strip. The enhanced mass loss helps illuminate the issue of infrared excess and the mass discrepancy found in Cepheids.Comment: 46 pages, 12 figures, 6 tables, ApJ accepte
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