The interstellar medium is enriched primarily by matter ejected from old, evolved stars. The outflows from these stars create spherical envelopes, which foster gas-phase chemistry. The chemical ...complexity in circumstellar shells was originally thought to be dominated by the elemental carbon to oxygen ratio. Observations have suggested that envelopes with more carbon than oxygen have a significantly greater abundance of molecules than their oxygen-rich analogues. Here we report observations of molecules in the oxygen-rich shell of the red supergiant star VY Canis Majoris (VY CMa). A variety of unexpected chemical compounds have been identified, including NaCl, PN, HNC and HCO+. From the spectral line profiles, the molecules can be distinguished as arising from three distinct kinematic regions: a spherical outflow, a tightly collimated, blue-shifted expansion, and a directed, red-shifted flow. Certain species (SiO, PN and NaCl) exclusively trace the spherical flow, whereas HNC and sulphur-bearing molecules (amongst others) are selectively created in the two expansions, perhaps arising from shock waves. CO, HCN, CS and HCO+ exist in all three components. Despite the oxygen-rich environment, HCN seems to be as abundant as CO. These results suggest that oxygen-rich shells may be as chemically diverse as their carbon counterparts.
Millimeter-wave observations of PN, CP, and HCP have been carried out toward circumstellar envelopes of evolved stars using the Arizona Radio Observatory (ARO). HCP and PN have been identified in the ...carbon-rich source CRL 2688 via observations at 1 mm using the Submillimeter Telescope (SMT) and 2-3 mm with the Kitt Peak 12 m. An identical set of measurements were carried out toward IRC +10216, as well as observations of CP at 1 mm. PN was also observed toward VY Canis Majoris (VY CMa), an oxygen-rich supergiant star. The PN and HCP line profiles in CRL 2688 and IRC +10216 are roughly flat topped, indicating unresolved, optically thin emission; CP, in contrast, has a distinct 'U' shape in IRC +10216. Modeling of the line profiles suggests abundances, relative to H sub(2), of image and image in CRL 2688, about an order of magnitude higher than in IRC +10216. In VY CMa, f(PN) is image. The data in CRL 2688 and IRC +10216 are consistent with LTE formation of HCP and PN in the inner envelope, as predicted by theoretical calculations, with CP a photodissociation product at larger radii. The observed abundance of PN in VY CMa is a factor of 100 higher than LTE predictions. In IRC +10216, the chemistry of HCP/CP mimics that of HCN/CN and suggests an N sub(2) abundance of image. The chemistry of phosphorus appears active in both carbon- and oxygen-rich envelopes of evolved stars.
A comprehensive study of glycolaldehyde (CH sub(2)OHCHO) has been conducted at 2 and 3 mm toward Sgr B2(N) using the Arizona Radio Observatory 12 m telescope. Forty favorable transitions of this ...species were observed in the range 68-169 GHz. Emission on the 20-70 mK level was detected at frequencies of 38 of these lines, including all transitions arising from the K sub(a) = 0, 1, and 2 ladders. The two transitions not detected were weak and originate in the less populated K sub(a) = 3 levels. Twenty-one percent of the detected lines are distinct, individual features. The remaining transitions are either contaminated by emission from abundant molecules or blended with equivalently weak features. The unblended transitions indicate V sub(LSR) = 62.3 2.4 km s super(-1) and V sub(1/2) = 8.3 c 3.4 km s super(-1), line parameters characteristic of organic species in Sgr B2(N). A rotational diagram yields a column density of 5.9 x 10 super(13) cm super(-2) for glycolaldehyde, suggesting a fractional abundance of f(H sub(2)) = 5.9 x 10 super(-11). Observations of formaldehyde toward Sgr B2(N) suggest that H sub(2)CO and CH sub(2)OHCHO arise from the same gas with an abundance ratio of 61/27. H sub(2)CO may function as the precursor to glycolaldehyde in a gas-phase "formose" reaction. These observations, combined with past results of Hollis et al., provide convincing evidence for the presence of glycolaldehyde in the ISM. This study suggests that an extensive, self-consistent data set is necessary to identify large organic species in interstellar gas.
Abstract
We present the ‘dark-PMT’, a novel detector concept based around a target made of vertically-aligned carbon nanotubes. The detector is sensitive to electron recoils induced by sub-GeV dark ...matter, and is expected to have directional sensitivity and to be unaffected by thermal noise, even at room temperature. The key feature is that nanotubes are made of graphene, which is a two-dimensional material: therefore, if a dark matter particle transfers enough energy to an electron in the carbon lattice to overcome the work function (4.7 eV), the electron will be ejected directly into the vacuum. Because of the strong density anisotropy of nanotubes, the electrons will be capable of leaving the target, without being reabsorbed, if travelling in the direction of the tube axes. The electrons will then be accelerated, and reach an energy of 5 keV before hitting an electron sensor. We report on the most recent advancements towards the construction of a dark-PMT: a novel, state-of-the-art facility for nanotube synthesis has been recently installed in Rome, and it is being used to produce high-quality nanotubes; and detailed characterizations of silicon sensors with keV electrons have been performed.
L183 (L134N) revisited. III. The gas depletion PAGANI, L; PARDO, J.-R; APPONI, A. J ...
Astronomy and astrophysics (Berlin),
2005, 2005-01-00, 20050101, Letnik:
429, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present a detailed study of the gas depletion in L183 (= L134N) for a set of important species, namely, CO, CS, SO, N sub(2)H super(+) and NH sub(3). We show that all these species are depleted at ...some level. This level seems to depend mostly on a density threshold rather than on dust opacity. Therefore UV shielding would not be a main factor in the triggering of depletion. Our data suggest that CO, CS and SO depletion happen at densities of similar to 3 x 10 super(4) cm super(-3), while N sub(2)H super(+) and NH sub(3) seem to deplete at densities close to 10 super(6) cm super(-3). The latter result is consistent with the Bergin & Langer (1997, ApJ, 486, 316) polar (H sub(2)O) ice case but not with the more recent models of Aikawa et al. (2003, ApJ, 593, 906). CS depletion occurs much below its (J:2-1) critical density, (7 x 10 super(5) cm super(-3)) and therefore makes this species unsuitable to study the density structure of many dark cloud cores.
The NaCl molecule has been observed in the circumstellar envelopes of VY Canis Majoris (VY CMa) and IK Tauri (IK Tau)--the first identifications of a metal refractory in oxygen-rich shells of evolved ...stars. Five rotational transitions of NaCl at 1 and 2 mm were detected toward VY CMa and three 1 mm lines were observed toward IK Tau, using the telescopes of the Arizona Radio Observatory. In both objects, the line widths of the NaCl profiles were extremely narrow relative to those of other molecules, indicating that sodium chloride has not reached the terminal outflow velocity in either star, likely a result of early condensation onto grains. Modeling the observed spectra suggests abundances, relative to H sub(2), of f similar to 5 x 10 super(-9) in VY CMa and f similar to 4 x 10 super(-9) in IK Tau, with source sizes of 0.5" and 0.3", respectively. The extent of these sources is consistent with the size of the dust acceleration zones in both stars. NaCl therefore appears to be at least as abundant in O-rich shells as compared to C-rich envelopes, where f similar to (0.2-2) x 10 super(-9), although It appears to condense out earlier in the O-rich case. Chemical equilibrium calculations indicate that NaCl is the major carrier of sodium at T similar to 1100 K for oxygen-rich stars, with predicted fractional abundances in good agreement with the observations. These measurements suggest that crystalline salt may be an important condensate for sodium in both C- and O-rich circumstellar shells.