Low-resolution intensity spectra of Earth's atmosphere obtained from space reveal strong signatures of life ('biosignatures'), such as molecular oxygen and methane with abundances far from chemical ...equilibrium, as well as the presence of a 'red edge' (a sharp increase of albedo for wavelengths longer than 700 nm) caused by surface vegetation. Light passing through the atmosphere is strongly linearly polarized by scattering (from air molecules, aerosols and cloud particles) and by reflection (from oceans and land). Spectropolarimetric observations of local patches of Earth's sky light from the ground contain signatures of oxygen, ozone and water, and are used to characterize the properties of clouds and aerosols. When applied to exoplanets, ground-based spectropolarimetry can better constrain properties of atmospheres and surfaces than can standard intensity spectroscopy. Here we report disk-integrated linear polarization spectra of Earthshine, which is sunlight that has been first reflected by Earth and then reflected back to Earth by the Moon. The observations allow us to determine the fractional contribution of clouds and ocean surface, and are sensitive to visible areas of vegetation as small as 10 per cent. They represent a benchmark for the diagnostics of the atmospheric composition, mean cloud height and surfaces of exoplanets.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Context.
Scattering processes in the atmospheres of planets cause characteristic features that can be particularly well observed in polarisation. For planet Earth, both molecular scattering ...(Rayleigh) and scattering by small particles (Mie) imprint specific signatures in its phase curve. Polarised phase curves allow us to infer physical and chemical properties of the atmosphere like the composition of the gaseous and liquid components, droplet sizes, and refraction indices.
Aims.
An unequivocal prediction of a liquid-water-loaded atmosphere is the existence of a rainbow feature at a scattering angle of around 138–144°. Earthshine allows us to observe the primary rainbow in linear polarisation.
Methods.
We observed polarisation spectra of Earthshine using FORS2 at the Very Large Telescope for phase angles from 33° to 65° (Sun–Earth–Moon angle). The spectra were used to derive the degree of polarisation in the
B
,
V
,
R
, and
I
passbands and the phase curve from 33° to 136°. The new observations extend to the smallest phases that can be observed from the ground.
Results.
The degree of polarisation of planet Earth is increasing for decreasing phase angles downwards of 45° in the
B
,
V
,
R
, and
I
passbands. From comparison of the phase curve observed with models of an Earth-type atmosphere we are able to determine the refractive index of water and to constrain the mean water droplet sizes to 6−7
μ
m. Furthermore, we can retrieve the mean cloud fraction of liquid water clouds to 0.3, and the mean optical depth of the water clouds to values between 10 and 20.
Conclusions.
Our observations allow us to discern two fundamentally different scattering mechanisms of the atmosphere of planet Earth: molecular and particle scattering. The physical and chemical properties can be retrieved with high fidelity through suitable inversion of the phase curve. Observations of polarimetric phase curves of planets beyond the Solar System shall be extremely valuable for a thorough characterisation of their atmospheres.
Context. Earthshine, i.e., sunlight scattered by Earth and back-reflected from the lunar surface to Earth, allows observations of the total flux and polarization of Earth with ground-based ...astronomical facilities on timescales from minutes to years. Like flux spectra, polarization spectra exhibit imprints of the atmospheric and surface properties of Earth. Earth’s polarization spectra may prove an important benchmark to constrain expected biosignatures of Earth-like planets observed with future telescopes. Aims. We derive the polarimetric phase curve of Earth from a statistically significant sample of Earthshine polarization spectra. The impact of changing Earth views on the variation of polarization spectra is investigated. Methods. We present a comprehensive set of spectropolarimetric observations of Earthshine as obtained by FORS2 at the Very Large Telescope for phase angles from 50° to 135° (Sun–Earth–Moon angle), covering a spectral range from 4300 to 9200 Å. The degree of polarization in the B, V, R, I passbands, the differential polarization vegetation index, and the equivalent width of the O2-A polarization band around 7600 Å are determined with absolute errors around 0.1% in the degree of polarization. Earthshine polarization spectra are corrected for the effect of depolarization introduced by backscattering on the lunar surface, introducing systematic errors on the order of 1% in the degree of polarization. Results. Distinct viewing sceneries such as observing the Atlantic or Pacific side in Earthshine yield statistically different phase curves. The equivalent width defined for the O2-A band polarization is found to vary from −50 to +20 Å. A differential polarized vegetation index is introduced and reveals a larger vegetation signal for those viewing sceneries that contain larger fractions of vegetated surface areas. We corroborate the observed correlations with theoretical models from the literature, and conclude that the vegetation red edge (VRE) is a robust and sensitive signature in polarization spectra of planet Earth. Conclusions. The overall behavior of polarization of planet Earth in the continuum and in the O2-A band can be explained by existing models. Biosignatures such as the O2-A band and the VRE are detectable in Earthshine polarization with a high degree of significance and sensitivity. An in-depth understanding of the temporal and spectral variability of Earthshine requires improved models of Earth’s biosphere, as a prerequisite to interpreting possible detections of polarized biosignatures in Earth-like exoplanets in the future.
We analyze high-resolution (Δv ≤ 10 km s−1) optical and infrared spectra covering the O i λ6300 and Ne ii 12.81 m lines from a sample of 31 disks in different evolutionary stages. Following work at ...optical wavelengths, we use Gaussian profiles to fit the Ne ii lines and classify them into high-velocity component (HVC) or low-velocity component (LVC) if the line centroid is more or less blueshifted than 30 km s−1 with respect to the stellar radial velocity, respectively. Unlike for the O i, where an HVC is often accompanied by an LVC, all 17 sources with an Ne ii detection have either an HVC or an LVC. Ne ii HVCs are preferentially detected toward high accretors ( M yr−1), while LVCs are found in sources with low , low O i luminosity, and large infrared spectral index (n13-31). Interestingly, the Ne ii and O i LVC luminosities display an opposite behavior with n13-31: as the inner dust disk depletes (higher n13-31), the Ne ii luminosity increases while the O i weakens. The Ne ii and O i HVC profiles are generally similar, with centroids and FWHMs showing the expected behavior from shocked gas in microjets. In contrast, the Ne ii LVC profiles are typically more blueshifted and narrower than the O i profiles. The FWHM and centroid versus disk inclination suggest that the Ne ii LVC predominantly traces unbound gas from a slow, wide-angle wind that has not lost completely the Keplerian signature from its launching region. We sketch an evolutionary scenario that could explain the combined O i and Ne ii results and includes screening of hard (∼1 keV) X-rays in inner, mostly molecular, MHD winds.
The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from ...the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Context. Ground-based observations of the Earthshine, i.e., the light scattered by Earth to the Moon, and then reflected back to Earth, simulate space observations of our planet and represent a ...powerful benchmark for the studies of Earth-like planets. Earthshine spectra are strongly linearly polarized, owing to scattering by molecules and small particles in the atmosphere of the Earth and surface reflection, and may allow us to measure global atmospheric and surface properties of planet Earth. Aims. We aim to interpret already published spectropolarimetric observations of the Earthshine by comparing them with new radiative transfer model simulations including a fully realistic three-dimensional (3D) surface-atmosphere model for planet Earth. Methods. We used the highly advanced Monte Carlo radiative transfer model MYSTIC to simulate polarized radiative transfer in the atmosphere of the Earth without approximations regarding the geometry, taking into account the polarization from surface reflection and multiple scattering by molecules, aerosol particles, cloud droplets, and ice crystals. Results. We have shown that Earth spectropolarimetry is highly sensitive to all these input parameters, and we have presented simulations of a fully realistic Earth atmosphere-surface model including 3D cloud fields and two-dimensional (2D) surface property maps. Our modeling results show that scattering in high ice water clouds and reflection from the ocean surface are crucial to explain the continuum polarization at longer wavelengths as has been reported in Earthshine observations taken at the Very Large Telescope in 2011 (3.8% and 6.6% at 800 nm, depending on which part of Earth was visible from the Moon at the time of the observations). We found that the relatively high degree of polarization of 6.6% can be attributed to light reflected by the ocean surface in the sunglint region. High ice-water clouds reduce the amount of absorption in the O2A band and thus explain the weak O2A band feature in the observations.
Linear broadband polarimetry is used to characterize the objects of our solar system, and has also been proposed as a diagnostic tool for the atmospheres of exo-solar planets. Homochirality ...characterizes life as we know it and induces circular polarization in the diffuse reflectance spectra of biotic material. Hence it has been suggested that circular polarimetry may be used as a remote sensing tool for the search of extra-terrestrial life. With this motivation in mind we have decided to explore the potential of both linear and circular spectropolarimetry as a diagnostic tool for remote sensing of biotic material. We have used the calibration unit of the EFOSC2 instrument of the La Silla Observatory to obtain low resolution, but high signal to noise circular and linear spectropolarimetric measurements of a number of inorganic and organic materials. We then compare our “laboratory data” with spectropolarimetric observations of atmosphere-less bodies of our solar system and of Earthshine obtained with instruments very similar to that one used for our laboratory samples. We conclude that linear polarization measurements are more suitable than circular polarization measurements for the characterization of planetary surfaces and atmospheres, and for the search of extra-terrestrial life.
ABSTRACT
Utilizing several instruments on 4–8 m telescopes, we have observed a large sample of objects in the mid-infrared (8–13 μm). These comprise a few evolved stars, multiple envelopes of ...embedded young stellar objects (YSOs) or compact H-II regions, and several sightlines through the interstellar medium (ISM). The latter is where dust resides – and is potentially modified – between its formation in evolved stellar outflows and deposition in molecular clouds. In most objects, we detect not only the well-known 9.7 μm absorption feature of amorphous silicates but also a second absorption band around 11.1 μm whose carrier is attributed to crystalline forsterite. We propose that crystalline silicates are essentially ubiquitous in the ISM and earliest phases of star formation, and are evolutionary precursors to T-Tauri and Herbig stars where such silicates have been commonly found. Modelling shows that in most YSOs, H-II regions and ISM cases, the forsterite mass fraction is between 1 and 2 per cent, suggesting that the younger phases inherit their abundance from the ISM. However, several sources show much stronger features (abundances ≥3 per cent). This suggests that significant processing, perhaps crystallization by thermal annealing, occurs early on in star formation. Most intriguing is the first detection of crystalline silicate in the diffuse ISM. We propose that our observed abundance is consistent with a mass fraction of crystalline silicates of 10–20 per cent injected into the ISM, along with commonly accepted lifetimes against their destruction, but only if cosmic ray-induced amorphization is insignificant over a few Giga years.
There are several ways planets can survive the giant phase of the host star, hence one can consider the case of Earth-like planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, ...a planet orbiting at 0.01 AU from the star would remain in the continuous habitable zone (CHZ) for about 8 Gyr. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 102 (104) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a powerful tool to detect close-in planets around white dwarfs. Multi-band polarimetry would also allow one to reveal the presence of a planet atmosphere, even providing a first characterisation. With current facilities a super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue. Preliminary habitability study show also that photosynthetic processes can be sustained on Earth-like planets orbiting CWDs and that the DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence white dwarfs are compatible with the persistence of complex life from the perspective of UV irradiation.
The past decade has seen increasing efforts in detecting and characterising exoplanets using high-contrast imaging in the near- and mid-infrared, which is the optimal wavelength domain for studying ...old, cold planets. In this work, we present deep adaptive optics imaging observations of the nearby Sun-like star
ϵ
Ind A with the NaCo (
L
′) and NEAR (10–12.5 microns) instruments at VLT in an attempt to directly detect its planetary companion, whose presence has been indicated from radial velocity (RV) and astrometric trends. We derive brightness limits from the non-detection of the companion with both instruments and interpret the corresponding sensitivity in mass based on both cloudy and cloud-free atmospheric and evolutionary models. For an assumed age of 5 Gyr for the system, we get detectable mass limits as low as 4.4
M
J
in NaCo
L
′ and 8.2
M
J
in NEAR bands at 1.5′′ from the central star. If the age assumed is 1 Gyr, we reach even lower mass limits of 1.7
M
J
in NaCo
L
′ and 3.5
M
J
in NEAR bands at the same separation. However, based on the dynamical mass estimate (3.25
M
J
) and ephemerides from astrometry and RV, we find that the non-detection of the planet in these observations puts a constraint of 2 Gyr on the lower age limit of the system. NaCo offers the highest sensitivity to the planetary companion in these observations, but the combination with the NEAR wavelength range adds a considerable degree of robustness against uncertainties in the atmospheric models. This underlines the benefits of including a broad set of wavelengths for the detection and characterisation of exoplanets in direct imaging studies.
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