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Search Results: 1 - 10 of 223695 matches for " R. Bachiller "
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The chemistry of compact planetary nebulae
E. Josselin,R. Bachiller
Physics , 2002, DOI: 10.1051/0004-6361:20021516
Abstract: We report high-sensitivity millimetre observations of several molecular species (13CO, HCN, HNC, CN, HCO+ and N2H+) in a sample of compact planetary nebulae. Some species such as HCO+ and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds. We have estimated the following average values for the column densities ratios: CN/HCN~2.6, HCO+/HCN~0.5, and HNC/HCN~0.4. Thus, the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such as the Ring, M4--9, or the Helix). From observations of the CO isotopomers, we have estimated that the 12C/13C ratio is in the range 10 ~< 12C/13C ~< 40. These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes. The observed molecular abundances are compared to very recent modelling work, and we conclude that the observations are well explained, in general terms, by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value. Thus, our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars. The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope. Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models.
Dust emission from young outflows: the case of L1157
F. Gueth,R. Bachiller,M. Tafalla
Physics , 2003, DOI: 10.1051/0004-6361:20030259
Abstract: We present new high-sensitivity 1.3 mm bolometer observations of the young outflow L1157. These data show that the continuum emission arises from four distinct components: a circumstellar disk, a protostellar envelope, an extended flattened envelope --the dense remnant of the molecular cloud in which the protostar was formed--, and the outflow itself, which represents ~20% of the total flux. The outflow emission exhibits two peaks that are coincident with the two strong shocks in the southern lobe of L1157. We show that the mm continuum is dominated by thermal dust emission arising in the high velocity material. The spectral index derived from the new 1.3 mm data and 850 mu observations from Shirley et al. (2000), is ~5 in the outflow, significantly higher than in the protostellar envelope (~3.5). This can be explained by an important line contamination of the 850 mu map, and/or by different dust characteristics in the two regions, possibly smaller grains in the post-shocks regions of the outflow. Our observations show that bipolar outflows can present compact emission peaks which must not be misinterpreted as protostellar condensations when mapping star forming regions.
JM Eiros,MR Bachiller,JM Pinilla,R Ortiz de Lejarazu
Electronic Journal of Biomedicine , 2005,
Measurements of the 12C/13C ratio in Planetary Nebulae and implications for stellar evolution
F. Palla,D. Galli,R. Bachiller,M. Perez Gutierrez
Physics , 1997,
Abstract: We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebuale (PNe) by means of mm-wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition; the other group consists of 23 nebulae with rich molecular envelopes. We have determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have ratios well below the value predicted by standard evolutionary models (about 20), indicating that some extra-mixing process has occurred in these stars. We briefly discuss the implications of our results for standard and nonstandard stellar nucleosynthesis.
Measurements of 12C/13C ratio in planetary nebulae: implications on stellar and Galactic chemical evolution
F. Palla,R. Bachiller,L. Stanghellini,M. Tosi,D. Galli
Physics , 1999,
Abstract: We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebulae (PNe) by means of millimeter wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition by Balser et al. (1997); the other group consists of 22 nebulae with rich molecular envelopes. We have determined the carbon isotopic ratio in 14 objects, 9 of which are new detections. The results indicate a range of values of 12C/13C between 9 and 23. We estimate the mass of the progenitors of the PNe of our sample and combine this information with the derived 12C/13C isotopic ratios to test the predictions of stellar nucleosynthesis models. We find that the majority of PNe have isotopic ratios below the values expected from current standard asymptotic giant branch models in the mass range of interest. We suggest that the progenitors of the PNe must have undergone a non-standard mixing process during their red giant phase and/or asymptotic giant phase, resulting in a significant enhancement of the 13C abundance in the surface layers. Our study confirms a similar behaviour inferred from spectroscopic observations of field population II stars and globular cluster giants, and extends it to the final stages of stellar evolution. Finally, we discuss the implications of our results on models of Galactic chemical evolution of 3He and 12C/13C.
L483: A Protostar In Transition From Class 0 to Class I
M. Tafalla,P. C. Myers,D. Mardones,R. Bachiller
Physics , 2000,
Abstract: We present molecular-line observations toward the dense core in L483 and its bipolar outflow powered by the Class 0 object IRAS 18148-0440. CO (carbon monoxide) maps show that the outflow is well collimated and asymmetric, and that its gas is warmer than the surrounding cloud by at least a factor of 2. H2CO (formaldehyde) lines toward the outflow show prominent high-velocity wings and evidence for an H2CO abundance enhancement of a factor of 20. At ambient velocities, these lines show strong self-absorption and a brighter blue peak, a characteristic signature of inward motions. Finally, and in contrast with the outflows from other Class 0 objects, the CH3OH (methanol) lines in L483 do not show high velocity wings, and no evidence for abundance enhancement is found in this molecule or in SiO (silicon monoxide). Comparing the physical and chemical properties of the outflow in L483 with those of other outflows from Class 0 and Class I sources, we find that the L483 outflow is somewhat intermediate between these types. This suggests that the L483 central source has already started its transition between Class 0 and Class I, and that its mixed properties illustrate how this transition occurs.
Molecular ions in the protostellar shock L1157-B1
L. Podio,B. Lefloch,C. Ceccarelli,C. Codella,R. Bachiller
Physics , 2014, DOI: 10.1051/0004-6361/201322928
Abstract: We perform a complete census of molecular ions with an abundance larger than 1e-10 in the protostellar shock L1157-B1 by means of an unbiased high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means of an LVG radiative transfer code the gas physical conditions and fractional abundances of molecular ions are derived. The latter are compared with estimates of steady-state abundances in the cloud and their evolution in the shock calculated with the chemical model Astrochem. We detect emission from HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with respect to systemic, has a width of ~ 4-8 km/s, and is associated with the outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+ abundances are in agreement with steady-state abundances in the cloud and with their evolution in the compressed and heated gas in the shock for cosmic rays ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances, instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the other hand they are strongly enhanced on timescales shorter than the shock age (~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are effective shock tracers and can be used to infer the amount of CO2 and sulphur-bearing species released from dust mantles in the shock. The observed HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur carrier on grain mantles. However, the OCS abundance required to fit the observations is 1-2 orders of magnitude larger than observed. Further studies are required to fully understand the chemistry of sulphur-bearing species.
The origin of the HH7-11 outflow
R. Bachiller,F. Gueth,S. Guilloteau,M. Tafalla,A. Dutrey
Physics , 2000,
Abstract: New, high-sensitivity interferometric CO J=2-1 observations of the HH 7-11 outflow show that despite previous doubts, this system is powered by the Class I source SVS 13. The molecular outflow from SVS 13 is formed by a shell with a large opening angle at the base, which is typical of outflows from Class I sources, but it also contains an extremely-high-velocity jet composed of ``molecular bullets'', which is more typical of Class 0 outflows. This suggests that SVS 13 could be a very young Class I, which still keeps some features of the previous evolutionary stage. We briefly discuss the nature of some sources in the SVS 13 vicinity which are emitters of cm-wave continuum, but have no counterpart at mm wavelengths.
A molecular survey of outflow gas: velocity-dependent shock chemistry and the peculiar composition of the EHV gas
M. Tafalla,J. Santiago-Garcia,A. Hacar,R. Bachiller
Physics , 2010, DOI: 10.1051/0004-6361/201015158
Abstract: (Abridged) We present a molecular survey of the outflows powered by L1448-mm and IRAS 04166+2706, two sources with prominent wing and extremely high velocity (EHV) components in their CO spectra. The molecular composition of the two outflows presents systematic changes with velocity that we analyze by dividing the outflow in three chemical regimes, two of them associated with the wing component and the other the EHV gas. The analysis of the two wing regimes shows that species like H2CO and CH3OH favor the low-velocity gas, while SiO and HCN are more abundant in the fastest gas. We also find that the EHV regime is relatively rich in O-bearing species, as is not only detected in CO and SiO (already reported elsewhere), but also in SO, CH3OH, and H2CO (newly reported here), with a tentative detection in HCO+. At the same time, the EHV regime is relatively poor in C-bearing molecules like CS and HCN. We suggest that this difference in composition arises from a lower C/O ratio in the EHV gas. The different chemical compositions of the wing and EHV regimes suggest that these two outflow components have different physical origins. The wing component is better explained by shocked ambient gas, although none of the existing shock models explains all observed features. The peculiar composition of the EHV gas may reflect its origin as a dense wind from the protostar or its surrounding disk.
Cometary molecular clouds around RNO6. On-going star formation near the double cluster h and chi Persei
R. Bachiller,A. Fuente,M. S. N. Kumar
Physics , 2001, DOI: 10.1051/0004-6361:20011506
Abstract: We present molecular line observations of the star-forming cloud around RNO6 along with a newly discovered nearby molecular cloud that we name RNO6NW. Both clouds display striking similarities in their cometary structures and overall kinematics. By using 13CO line observations, we estimate that these clouds have similar sizes (~4.5 pc) and masses (~200 solar masses). Both molecular clouds RNO6 and RNO6NW are active in star formation. From new high resolution near-IR narrowband images, we confirm that RNO6 hosts an embedded IR cluster that includes a Herbig Be star. A conspicuous H2 filament is found to delineate the dense cometary head of the globule. RNO6NW hosts at least two IR sources and a bipolar molecular outflow of ~0.9 pc of length and ~0.5 solar masses. We show that the cometary structure of both clouds has been created by the UV radiation from numerous OB stars lying ~1.5 degree to the north. Such OB stars are associated with the double cluster h and chi Persei, and are probably members of the PerOB1 association. Thus star formation inside these clouds has been very likely triggered by the Radiation Driven Implosion (RDI) mechanism. From comparison to RDI theoretical models, we find that the similar kinematics and morphology of both clouds is well explained if they are at a re-expansion phase. Triggered sequential star formation also explains the observed spatial distribution of the members of the near-IR cluster inside the RNO6 cloud, and the morphology of the H2 filament. We conclude that the RNO6 and RNO6NW clouds are high-mass counterparts to the cometary globules of smaller masses which have been studied up to now. Thus our observations demonstrate that the RDI mechanism can produce, not only low mass stars in small globules, but also intermediate mass stars and clusters in massive clouds.
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