| 1. |
- Alcolea, J., et al.
(författare)
-
HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star
- 2013
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559, s. (article nr.) A93-
-
Tidskriftsartikel (refereegranskat)abstract
- Aims.The study of the molecular gas in the circumstellar envelopes of evolved stars is normally undertaken by observing lines ofCO (and other species) in the millimetre-wave domain. In general, the excitation requirements of the observed lines are low at thesewavelengths, and therefore these observations predominantly probe the cold outer envelope while studying the warm inner regions ofthe envelopes normally requires sub-millimetre (sub-mm) and far-infrared (FIR) observational data.Methods.To gain insight into the physical conditions and kinematics of the warm (100–1000 K) gas around the red hyper-giantVY CMa, we performed sensitive high spectral resolution observations of molecular lines in the sub-mm/FIR using the HIFI instru-ment of theHerschelSpace Observatory. We observed CO, H2O, and other molecular species, sampling excitation energies from afew tens to a few thousand K. These observations are part of theHerschelguaranteed time key program HIFISTARS.Results.We detected theJ=6–5,J=10–9, andJ=16–15 lines of12CO and13CO at∼100, 300, and 750 K above the groundstate (and the13COJ=9–8 line). These lines are crucial for improving the modelling of the internal layers of the envelope aroundVY CMa. We also detected 27 lines of H2O and its isotopomers, and 96 lines of species such as NH3,SiO,SO,SO2HCN, OH andothers, some of them originating from vibrationally excited levels. Three lines were not unambiguously assigned.Conclusions.Our observations confirm that VY CMa’s envelope must consist of two or more detached components. The molecularexcitation in the outer layers is significantly lower than in the inner ones, resulting in strong self-absorbed profiles in molecular linesthat are optically thick in this outer envelope, for instance, low-lying lines of H2O. Except for the most abundant species, CO and H2O,most of the molecular emission detected at these sub-mm/FIR wavelengths arise from the central parts of the envelope. The spectrumof VY CMa is very prominent in vibrationally excited lines, which are caused by the strong IR pumping present in the central regions.Compared with envelopes of other massive evolved stars, VY CMa’s emission is particularly strong in these vibrationally excitedlines, as well as in the emission from less abundant species such as H13CN, SO, and NH3.
|
|
| 2. |
|
|
| 3. |
- Bjerkeli, Per, 1977, et al.
(författare)
-
H2O line mapping at high spatial and spectral resolution Herschel observations of the VLA 1623 outflow
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 546, s. Article Number: A29 -
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Apart from being an important coolant, water is known to be a tracer of high-velocity molecular gas. Recent models predict relatively high abundances behind interstellar shockwaves. The dynamical and physical conditions of the water emitting gas, however, are not fully understood yet. Using the Herschel Space Observatory, it is now possible to observe water emission from supersonic molecular outflows at high spectral and spatial resolution. Several molecular outflows from young stars are currently being observed as part of the WISH (Water In Star-forming regions with Herschel) key program. Aims. We aim to determine the abundance and distribution of water, its kinematics, and the physical conditions of the gas responsible for the water emission. The observed line profile shapes help us understand the dynamics in molecular outflows. Methods. We mapped the VLA1623 outflow, in the ground-state transitions of o-H2O, with the HIFI and PACS instruments. We also present observations of higher energy transitions of o-H2O and p-H2O obtained with HIFI and PACS towards selected outflow positions. From comparison with non-LTE radiative transfer calculations, we estimate the physical parameters of the water emitting regions. Results. The observed water emission line profiles vary over the mapped area. Spectral features and components, tracing gas in different excitation conditions, allow us to constrain the density and temperature of the gas. The water emission originates in a region where temperatures are comparable to that of the warm H-2 gas (T greater than or similar to 200 K). Thus, the water emission traces a gas component significantly warmer than the gas responsible for the low-J CO emission. The water column densities at the CO peak positions are low, i.e. N(H2O) similar or equal to (0.03-10) x 10(14) cm(-2). Conclusions. The water abundance with respect to H-2 in the extended outflow is estimated at X(H2O)
|
|
| 4. |
- Bujarrabal, V., et al.
(författare)
-
Herschel/HIFI observations of high-J CO transitions in the protoplanetary nebula CRL 618
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L3-
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas components in the protoplanetary nebula CRL 618. These components are particularly important for understanding the evolution of the nebula. Methods. We performed Herschel/HIFI observations of several CO lines in the far-infrared/sub-mm in the protoplanetary nebula CRL 618. The high spectral resolution provided by HIFI allows measurement of the line profiles. Since the dynamics and structure of the nebula is well known from mm-wave interferometric maps, it is possible to identify the contributions of the different nebular components (fast bipolar outflows, double shells, compact slow shell) to the line profiles. The observation of these relatively high-energy transitions allows an accurate study of the excitation conditions in these components, particularly in the warm ones, which cannot be properly studied from the low-energy lines. Results. The (CO)-C-12 J = 16-15, 10-9, and 6-5 lines are easily detected in this source. Both (CO)-C-13 J = 10-9 and 6-5 are also detected. Wide profiles showing spectacular line wings have been found, particularly in (CO)-C-12 J = 16-15. Other lines observed simultaneously with CO are also shown. Our analysis of the CO high-J transitions, when compared with the existing models, confirms the very low expansion velocity of the central, dense component, which probably indicates that the shells ejected during the last AGB phases were driven by radiation pressure under a regime of maximum transfer of momentum. No contribution of the diffuse halo found from mm-wave data is identified in our spectra, because of its low temperature. We find that the fast bipolar outflow is quite hot, much hotter than previously estimated; for instance, gas flowing at 100 km s(-1) must have a temperature higher than similar to 200 K. Probably, this very fast outflow, with a kinematic age
|
|
| 5. |
- Bujarrabal, V., et al.
(författare)
-
Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537, s. Article Number: A8-
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas in protoplanetary nebulae and young planetary nebulae (PPNe, PNe). The information that the observations of the different components deliver is of particular importance for understanding the evolution of these objects. Methods. We performed Herschel/HIFI observations of intermediate-excitation molecular lines in the far-infrared/submillimeter range in a sample of ten nebulae. The high spectral resolution provided by HIFI allows the accurate measurement of the line profiles. The dynamics and evolution of these nebulae are known to result from the presence of several gas components, notably fast bipolar outflows and slow shells (that often are the fossil AGB shells), and the interaction between them. Because of the diverse kinematic properties of the different components, their emissions can be identified in the line profiles. The observation of these high-energy transitions allows an accurate study of the excitation conditions, particularly in the warm gas, which cannot be properly studied from the low-energy lines. Results. We have detected FIR/sub-mm lines of several molecules, in particular of (CO)-C-12, (CO)-C-13, and H2O. Emission from other species, like NH3, OH, (H2O)-O-18, HCN, SiO, etc., has been also detected. Wide profiles showing sometimes spectacular line wings have been found. We have mainly studied the excitation properties of the high-velocity emission, which is known to come from fast bipolar outflows. From comparison with general theoretical predictions, we find that CRL 618 shows a particularly warm fast wind, with characteristic kinetic temperature T-k greater than or similar to 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, T-k similar to 30 K. Other nebulae, like CRL 2688, show intermediate temperatures, with characteristic values around 100 K. We also discuss how the complex structure of the nebulae can affect our estimates, considering two-component models. We argue that the differences in temperature in the different nebulae can be caused by cooling after the gas acceleration (that is probably caused by shocks); for instance, CRL 618 is a case of very recent acceleration, less than similar to 100 yr ago, while the fast gas in OH 231.8+4.2 was accelerated similar to 1000 yr ago. We also find indications that the densest gas tends to be cooler, which may be explained by the expected increase of the radiative cooling efficiency with the density.
|
|
| 6. |
- Cernicharo, J., et al.
(författare)
-
Discovery of time variation of the intensity of molecular lines in IRC+10216 in the submillimeter and far-infrared domains
- 2014
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 796:1
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species toward the archetypical circumstellar envelope of IRC+10216. The observations have been carried out with the Heterodyne Instrument for the Far-Infrared (HIFI) instrument on board Herschel and with the IRAM30 m telescope. They cover several observing periods spreading over three years. The line intensity variations for molecules produced in the external layers of the envelope most likely result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations must take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The submillimeter and far infrared lines of asymptotic giant branch stars can no longer be considered as safe intensity calibrators.
|
|
| 7. |
- Chen, Jo-Hsin, et al.
(författare)
-
HERSCHEL HIFI OBSERVATIONS OF O-2 TOWARD ORION: SPECIAL CONDITIONS FOR SHOCK ENHANCED EMISSION
- 2014
-
Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 793:2, s. Article nr. 111 -
-
Tidskriftsartikel (refereegranskat)abstract
- We report observations of molecular oxygen (O-2) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O-2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km s(-1) with line widths similar to 3 km s(-1); however, the transition at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O-2 emission is similar or equal to 9" (6 x 10(16) cm) in size, and is located close to the H-2 Peak 1 position (where vibrationally excited H-2 emission peaks), and not at Peak A, 23" away. The peak O-2 column density is similar to 1.1 x 10(18) cm(-2). The line velocity is close to that of the 621 GHz water maser emission found in this portion of the Orion Molecular Cloud, and having a shock with velocity vector lying nearly in the plane of the sky is consistent with producing maximum maser gain along the line of sight. The enhanced O-2 abundance compared to that generally found in dense interstellar clouds can be explained by passage of a low-velocity C shock through a clump with preshock density 2 x 10(4) cm(-3), if a reasonable flux of UV radiation is present. The postshock O-2 can explain the emission from the source if its line-of-sight dimension is similar or equal to 10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O-2 emission has not been detected in the cores of other massive star-forming molecular clouds.
|
|
| 8. |
- Comito, C., et al.
(författare)
-
Herschel observations of deuterated water towards Sgr B2(M)
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521, s. L38-
-
Tidskriftsartikel (refereegranskat)abstract
- Observations of HDO are an important complement for studies of water, because they give strong constraints on the formation processes - grain surfaces versus energetic process in the gas phase, e. g. in shocks. The HIFI observations of multiple transitions of HDO in Sgr B2(M) presented here allow the determination of the HDO abundance throughout the envelope, which has not been possible before with ground-based observations only. The abundance structure has been modeled with the spherical Monte Carlo radiative transfer code RATRAN, which also takes radiative pumping by continuum emission from dust into account. The modeling reveals that the abundance of HDO rises steeply with temperature from a low abundance (2.5 x 10(-11)) in the outer envelope at temperatures below 100 K through a medium abundance (1.5 x 10(-9)) in the inner envelope/outer core at temperatures between 100 and 200 K, and finally a high abundance (3.5 x 10(-9)) at temperatures above 200 K in the hot core.
|
|
| 9. |
- Decin, L., et al.
(författare)
-
Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L4-
-
Tidskriftsartikel (refereegranskat)abstract
- During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of (H2O)-O-16 and the rarer isotopologues (H2O)-O-17 and (H2O)-O-18 in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 x 10(-5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of (CO)-C-12, (CO)-C-13, (SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.
|
|
| 10. |
- Falgarone, E., et al.
(författare)
-
CH+(1-0) and 13CH+(1-0) absorption lines in the direction of massive star-forming regions
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1
-
Tidskriftsartikel (refereegranskat)abstract
- We report the detection of the ground-state rotational transition of the methylidyne cation CH+ and its isotopologue (CH+)-C-13 toward the remote massive star-forming regions W33A, W49N, and W51 with the HIFI instrument onboard the Herschel satellite. Both lines are seen only in absorption against the dust continuum emission of the star-forming regions. The CH+ absorption is saturated over almost the entire velocity ranges sampled by the lines-of-sight that include gas associated with the star-forming regions (SFR) and Galactic foreground material. The CH+ column densities are inferred from the optically thin components. A lower limit of the isotopic ratio [(CH+)-C-12]/[(CH+)-C-13]> 35.5 is derived from the absorptions of foreground material toward W49N. The column density ratio, N(CH+)/N(HCO+), is found to vary by at least a factor 10, between 4 and > 40, in the Galactic foreground material. Line-of-sight 12CH+ average abundances relative to total hydrogen are estimated. Their average value, N(CH+)/NH > 2.6 x 10(-8), is higher than that observed in the solar neighborhood and confirms the high abundances of CH+ in the Galactic interstellar medium. We compare this result to the predictions of turbulent dissipation regions (TDR) models and find that these high abundances can be reproduced for the inner Galaxy conditions. It is remarkable that the range of predicted N(CH+)/ N(HCO+) ratios, from 1 to similar to 50, is comparable to that observed.
|
|
