Monday, August 29, 2016

When has the Galaxy Actually Been Habitable?

The evolution of galaxy habitability

Authors:

Gobat et al

Abstract:

We combine a semi-analytic model of galaxy evolution with constraints on circumstellar habitable zones and the distribution of terrestrial planets to probe the suitability of galaxies of different mass and type to host habitable planets, as well as its evolution with time. We find that the fraction of stars with terrestrial planets in their habitable zone ("habitability") depends only weakly on galaxy mass, with a maximum around 4e10 Msun. We estimate that 0.7% of all stars in Milky Way type galaxies to host a terrestrial planet within their habitable zone, consistent with the value derived from Kepler observations. On the other hand, the habitability of passive galaxies is slightly but systematically higher, unless we assume an unrealistically high sensitivity of planets to supernovae. We find that the overall habitability of galaxies has not changed significantly in the last ~8 Gyr, with most of the habitable planets in local disk galaxies having formed ~1.5 Gyr before our own solar system. Finally, we expect that ~1.4e9 planets similar to present-day Earth have existed so far in our galaxy.

Reduced Activity And Large Particles From the Disintegrating Planet Candidate KIC 12557548b

Reduced Activity And Large Particles From the Disintegrating Planet Candidate KIC 12557548b

Authors:
Schlawin et al

Abstract:

The intriguing exoplanet candidate KIC 12557548b is believed to have a comet-like tail of dusty debris trailing a small rocky planet. The tail of debris scatters up to 1.3% of the stellar light in the Kepler observatory's bandpass (0.42 um to 0.9 um). Observing the tail's transit depth at multiple wavelengths can reveal the composition and particle size of the debris, constraining the makeup and lifetime of the sub-Mercury planet. Early dust particle size predictions from the scattering of the comet-like tail pointed towards a dust size of ~0.1 um for silicate compositions. These small particles would produce a much deeper optical transit depth than near-infrared transit depth. We measure a transmission spectrum for KIC 12557548b using the SpeX spectrograph (covering 0.8 um to 2.4 um) simultaneously with the MORIS imager taking r' (0.63 um) photometry on the Infrared Telescope Facility for eight nights and one night in H band (1.63 um) using the Wide-Field IR Camera at the Palomar 200-inch telescope. The infrared spectra are plagued by systematic errors, but we argue that sufficient precision is obtained when using differential spectroscopic calibration when combining multiple nights. The average differential transmission spectrum is flat, supporting findings that KIC 12557548b's debris is likely composed of larger particles greater than ~0.5 um for pyroxene and olivine and greater than ~0.2 um for iron and corundum. The r' photometric transit depths are all below the average Kepler value, suggesting that the observations occurred during a weak period or that the mechanisms producing optical broadband transit depths are suppressed.

Sunday, August 28, 2016

Exocometary gas in the HD 181327 debris ring

Exocometary gas in the HD 181327 debris ring

Authors:

Marino et al

Abstract:

An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now we only knew of cases around A stars. Here we present the first detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 +- 0.4 AU and a radial width of 23.2 +- 1.0 AU. At this frequency the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low level continuum emission beyond the main ring out to ~200 AU. We model the CO emission in the non-LTE regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2x10^-6 Mearth and 2.9x10^-6 Mearth, depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO2 cometary composition that is consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is shaping the dust distribution.

Volatile carbon locking and release in protoplanetary disks

Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546

Authors:

Kama et al

Abstract:

The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. We combine our recent detections of C0 in these disks with observations of other carbon reservoirs (CO, C+, C2H) and gas mass and warm gas tracers (HD, O0), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of ≤0.1M⊙. The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is at most moderately depleted compared to the interstellar medium, with [C]/[H]gas=(0.1−1.5)×10−4. HD 100546 itself is a ╬╗Bo\"{o}tis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H]gas=(0.2−5.0)×10−6 and C/O greater than 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H]gas in the outer disk of TW Hya and allows to make predictions for other systems.

Incidence of debris discs around FGK stars in the solar neighbourhood

Incidence of debris discs around FGK stars in the solar neighbourhood

Authors:

Montesinos et al

Abstract:

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. The full sample of 177 FGK stars with d less than 20 pc proposed for the DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE photometry, were obtained. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the DEBRIS consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age.

The subsample of 105 stars with d less than 15 pc containing 23 F, 33 G and 49 K stars, is complete for F stars, almost complete for G stars and contains a substantial number of K stars to draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type 0.26 (6 objects with excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49 K stars), the fraction for all three spectral types together being 0.22 (23 out of 105 stars). Uncertainties corresponding to a 95% confidence level are given in the text for all these numbers. The medians of the upper limits of L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K); the lowest values being around 4.0E-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.

Saturday, August 27, 2016

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

Authors:


Barenfeld et al

Abstract:

We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and 12CO J=3−2 line fluxes of disks around low mass (0.14−1.66 M⊙) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area (≲40 AU), or if the CO is heavily depleted by a factor of at least ∼1000 relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of Mdust∝M1.67±0.37∗. Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of log(Mdust/M∗) between Taurus and Upper Sco is 0.64±0.09, such that Mdust/M∗ is lower in Upper Sco by a factor of ∼4.5.

The Smoking gun of X-ray Photoevaporation for Protoplanetary Disks

Blueshifted [OI] lines from protoplanetary discs: the smoking gun of X-ray photoevaporation.

Authors:

Ercolanno et al

Abstract:

Photoevaporation of protoplanetary discs by high energy radiation from the central young stellar object is currently the favourite model to explain the sudden dispersal of discs from the inside out. While several theoretical works have provided a detailed pictured of this process, the direct observational validation is still lacking. Emission lines produced in these slow moving protoplanetary disc winds may bear the imprint of the wind structure and thus provide a potential diagnostic of the underlying dispersal process. In this paper we primarily focus on the collisionally excited neutral oxygen line at 6300A. We compare our models predictions to observational data and demonstrate a thermal origin for the observed blueshifted low-velocity component of this line from protoplanetary discs. Furthermore our models show that while this line is a clear tell-tale-sign of a warm, quasi-neutral disc wind, typical of X-ray photoevaporation, its strong temperature dependence makes it unsuitable to measure detailed wind quantities like mass-loss-rate.