68 research outputs found
The way forward
For the last few decades the study of disks around stars young and old and of
different types have progressed significantly. During the same time a
completely new discipline, the study of exoplanets, planets orbiting stars
other than our Sun, have emerged. Both these fields, which are interconnected,
have benefited from the development of new instrumentation, and especially by
telescopes and detectors deployed in space. In this chapter we are describing
the state of the art of such instruments and make an inventory of what is being
currently developed. We also state some of the requirements of the next steps
and what type of instruments will lead the way forward.Comment: 25 pages, 4 figure
Oxygen in dense interstellar gas : the oxygen abundance of the star forming core ρ Ophiuchi A
Context: Oxygen is the third most abundant element in the universe, but its chemistry in the interstellar medium is still not understood well. Aims: To critically examine the entire oxygen budget, we initially attempt to estimate the abundance of atomic oxygen, O, in the only region where molecular oxygen, O{2}, has been detected to date. Methods: We analysed ISOCAM-CVF spectral image data toward ρ Oph A to derive the temperatures and column densities of H{2} at the locations of ISO-LWS observations of two [O I] ^3P{J} lines. The intensity ratios of the (J = 1-2) 63 μm to (J = 0-1) 145 μm lines largely exceed ten, attesting to these lines being optically thin. This is confirmed by radiative transfer calculations, making these lines suitable for abundance determinations. For that purpose, we calculated line strengths and compared them to the LWS observations. Results: Excess [O I] emission is observed to be associated with the molecular outflow from VLA 1623. For this region, we determine the physical parameters, T and N(H{2}), from the CAM observations, and the gas density, n(H{2}), is determined from the flux ratio of the [O i] 63 μm and [O i] 145 μm lines. For the oxygen abundance, our analysis essentially leads to three possibilities: (1) extended low-density gas with standard ISM O-abundance, (2) compact high-density gas with standard ISM O-abundance, and (3) extended high-density gas with reduced oxygen abundance, [O/H] 2 × 10-5. Conclusions: As option (1) disregards valid [O i] 145 μm data, we do not find it very compelling; instead, we favour option (3), as lower abundances are expected as a result of chemical cloud evolution, but we are not able to dismiss option (2) entirely. Observations at higher angular resolution than offered by the LWS are required to decide between these possibilities
Widespread atomic gas emission reveals the rotation of the beta Pictoris disk
We present high resolution Na I D spectroscopy of the beta Pic disk, and the
resonantly scattered sodium emission can be traced from less than 30 AU to at
least 140 AU from the central star. This atomic gas is co-existent with the
dust particles, suggestive of a common origin or source. The disk rotates
towards us in the south-west and away from us in the north-east. The velocity
pattern of the gas finally provides direct evidence that the faint linear
feature seen in images of the star is a circumstellar disk in Keplerian
rotation. From modelling the spatial distribution of the Na I line profiles we
determine the effective dynamical mass to be 1.40 +/- 0.05 M_sun, which is
smaller than the stellar mass, 1.75 M_sun. We ascribe this difference to the
gravity opposing radiation pressure in the Na I lines. We argue that this is
consistent with the fact that Na is nearly completely ionised throughout the
disk (Na I/Na < 10^-4). The total column density of sodium gas is N(Na) = 10^15
cm^-2.Comment: 9 pages, including 6 figs (fig.1 in colour). Accepted by ApJ
Gas and dust in the star-forming region ρ Oph A ∗, ∗∗, ∗∗∗: The dust opacity exponent β and the gas-to-dust mass ratio g2d
© ESO, 2015. Aims. We aim at determining the spatial distribution of the gas and dust in star-forming regions and address their relative abundances in quantitative terms. We also examine the dust opacity exponent β for spatial and/or temporal variations. Methods. Using mapping observations of the very dense ρ Oph A core, we examined standard 1D and non-standard 3D methods to analyse data of far-infrared and submillimetre (submm) continuum radiation. The resulting dust surface density distribution can be compared to that of the gas. The latter was derived from the analysis of accompanying molecular line emission, observed with Herschel from space and with APEX from the ground. As a gas tracer we used N<inf>2</inf>H<sup>+</sup>, which is believed to be much less sensitive to freeze-out than CO and its isotopologues. Radiative transfer modelling of the N<inf>2</inf>H<sup>+</sup> (J = 3-2) and (J = 6-5) lines with their hyperfine structure explicitly taken into account provides solutions for the spatial distribution of the column density N(H<inf>2</inf>), hence the surface density distribution of the gas. Results. The gas-to-dust mass ratio is varying across the map, with very low values in the central regions around the core SM 1. The global average, = 88, is not far from the canonical value of 100, however. In ρ Oph A, the exponent β of the power-law description for the dust opacity exhibits a clear dependence on time, with high values of 2 for the envelope-dominated emission in starless Class -1 sources to low values close to 0 for the disk-dominated emission in Class III objects. β assumes intermediate values for evolutionary classes in between. Conclusions. Since β is primarily controlled by grain size, grain growth mostly occurs in circumstellar disks. The spatial segregation of gas and dust, seen in projection toward the core centre, probably implies that, like C<sup>18</sup>O, also N<inf>2</inf>H<sup>+</sup> is frozen onto the grains
Herschel Search for O_2 toward the Orion Bar
We report the results of a search for molecular oxygen (O_2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the H II region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O_2 NJ = 33-12 transition at 487 GHz and the 5_(4)-3_(4) transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3σ upper limits established here translate to a total line-of-sight O2 column density <1.5 × 10^(16) cm^(–2) for an emitting region whose temperature is between 30 K and 250 K, or <1 × 10^(16) cm^(–2) if the O_2 emitting region is primarily at a temperature of ≲100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O_2 column density is less than 4 × 10^(15) cm^(–2), a value that is below, and possibly well below, model predictions for gas with a density of 10^(4)-10^(5) cm^(–3) exposed to a far-ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on A V of the Bar is less than required for O_2 to reach peak abundance; (3) the O_2 emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O_2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams
Deciphering Spectral Fingerprints of Habitable Extrasolar Planets
In this paper we discuss how we can read a planets spectrum to assess its
habitability and search for the signatures of a biosphere. After a decade rich
in giant exoplanet detections, observation techniques have now reached the
ability to find planets of less than 10 MEarth (so called Super-Earths) that
may potentially be habitable. How can we characterize those planets and assess
if they are habitable? The new field of extrasolar planet search has shown an
extraordinary ability to combine research by astrophysics, chemistry, biology
and geophysics into a new and exciting interdisciplinary approach to understand
our place in the universe. The results of a first generation mission will most
likely result in an amazing scope of diverse planets that will set planet
formation, evolution as well as our planet in an overall context.Comment: 17 pages, 10 figures, Astrobiology, 10, 1, 201
Legislative History: An Act to Allow an Income Tax Credit for School Tuition (HP364)(LD 509)
https://digitalmaine.com/legishist118/1508/thumbnail.jp
Discovery of a New Companion and Evidence of a Circumprimary Disk: Adaptive Optics Imaging of the Young Multiple System VW Cha
Since a majority of young low-mass stars are members of multiple systems, the
study of their stellar and disk configurations is crucial to our understanding
of both star and planet formation processes. Here we present near-infrared
adaptive optics observations of the young multiple star system VW Cha. The
previously known 0.7 arcsec binary is clearly resolved already in our raw J and
K band images. We report the discovery of a new, faint companion to the
secondary, at an apparent separation of only 0.1 arcsec or 16 AU. Our
high-resolution photometric observations also make it possible to measure the
J-K colors of each of the three components individually. We detect an infrared
excess in the primary, consistent with theoretical models of a circumprimary
disk. Analytical and numerical calculations of orbital stability show that VW
Cha may be a stable triple system. Using models for the age and total mass of
the secondary pair, we estimate the orbital period to be 74 years. Thus,
follow-up astrometric observations might yield direct dynamical masses within a
few years, and constrain evolutionary models of low-mass stars. Our results
demonstrate that adaptive optics imaging in conjunction with deconvolution
techniques is a powerful tool for probing close multiple systems.Comment: 13 pages including 2 figures. To appear in Astrophysical Journal
Letter
Herschel Search for O2 Toward the Orion Bar
We report the results of a search for molecular oxygen (O2) toward the Orion
Bar, a prominent photodissociation region at the southern edge of the HII
region created by the luminous Trapezium stars. We observed the spectral region
around the frequency of the O2 N_J = 3_3 - 1_2 transition at 487 GHz and the
5_4 - 3_4 transition at 774 GHz using the Heterodyne Instrument for the Far
Infrared on the Herschel Space Observatory. Neither line was detected, but the
3sigma upper limits established here translate to a total line-of-sight O2
column density < 1.5 10^16 cm^-2 for an emitting region whose temperature is
between 30K and 250 K, or < 1 10^16 cm^-2 if the O2 emitting region is
primarily at a temperature of ~< 100 K. Because the Orion Bar is oriented
nearly edge-on relative to our line of sight, the observed column density is
enhanced by a factor estimated to be between 4 and 20 relative to the face-on
value. Our upper limits imply that the face-on O2 column density is less than 4
10^15 cm^-2, a value that is below, and possibly well below, model predictions
for gas with a density of 10^4 - 10^5 cm^-3 exposed to a far ultraviolet flux
10^4 times the local value, conditions inferred from previous observations of
the Orion Bar. The discrepancy might be resolved if: (1) the adsorption energy
of O atoms to ice is greater than 800 K; (2) the total face-on Av of the Bar is
less than required for O2 to reach peak abundance; (3) the O2 emission arises
within dense clumps with a small beam filling factor; or, (4) the face-on depth
into the Bar where O2 reaches its peak abundance, which is density dependent,
corresponds to a sky position different from that sampled by our Herschel
beams.Comment: 30 pages, 7 figures, 1 table. Accepted for publication in Ap
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