| 11. |
- Beltran, M. T., et al.
(författare)
-
Self-similarity of the magnetic field at different scales: The case of G31.41+0.31
- 2024
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 686
-
Tidskriftsartikel (refereegranskat)abstract
- Context . Dust polarization observations of the massive protocluster G31.41+0.31 carried out at similar to 1 '' (3750 au) resolution with the SMA at 870 mu m have revealed one of the clearest examples to date of an hourglass-shaped magnetic field morphology in the high-mass regime. Additionally, similar to O.'' 24 (similar to 900 au) resolution observations with ALMA at 1.3 mm have confirmed these results. The next step is to investigate whether the magnetic field maintains its hourglass-shaped morphology down to circumstellar scales. Aims . To study the magnetic field morphology toward the four (proto)stars A, B, C, and D contained in G31.41+0.31 and examine whether the self-similarity observed at core scales (1 '' and 0.'' 24 resolution) still holds at circumstellar scales, we carried out ALMA observations of the polarized dust continuum emission at 1.3 mm and 3.1 mm at an angular resolution of similar to 0.'' 068 (similar to 250 au), sufficient to resolve the envelope emission of the embedded protostars. Methods . We used ALMA to perform full polarization observations at 233 GHz (Band 6) and 97.5 GHz (Band 3) with a synthesized beam of 0.'' 072 x 0.'' 064. We carried out polarization observations at two different wavelengths to confirm that the polarization traces magnetically aligned dust grains and is not due to dust self-scattering. Results . The polarized emission and the direction of the magnetic field obtained at the two wavelengths are basically the same, except for an area between the embedded sources C and B. In such an area, the emission at 1.3 mm could be optically thick and affected by dichroic extinction. In the rest of the core, the similarity of the emission at the two wavelengths suggests that the polarized emission is due to magnetically aligned grains. The polarized emission has been successfully modeled with a poloidal field with a small toroidal component on the order of 10% of the poloidal component, with a position angle phi = -63 degrees, an inclination i = 50 degrees, and a mass-to-flux ratio lambda = 2.66. The magnetic field axis is oriented perpendicular to the NE-SW velocity gradient detected in the core. The strength of the plane-of-the-sky component of the mean magnetic field, estimated using both the Davis-Chandrasekhar-Fermi and the polarization-intensity gradient methods, is in the range similar to 10-80 mG, for a density range 1.4 x 10(7)-5 x 10(8) cm(-3). The mass-to-flux ratio is in the range lambda similar to 1.9-3.0, which suggests that the core is "supercritical". The polarization-intensity gradient method indicates that the magnetic field cannot prevent gravitational collapse inside the massive core. The collapse in the external part of the core is (slightly) sub-Alfvenic and becomes super-Alfvenic close to the center. Conclusions . Dust polarization measurements from large core scales to small circumstellar scales, in the hot molecular core G31.41+0.31 have confirmed the presence of a strong magnetic field with an hourglass-shaped morphology. This result suggests that the magnetic field could have a relevant role in regulating the star-forming process of massive stars at all scales, although it cannot prevent the collapse. However, it cannot be ruled out that the large opacity of the central region of the core may hinder the study of the magnetic field at circumstellar scales. Therefore, high-angular resolution observations at longer wavelengths, tracing optically thinner emission, are needed to confirm this self-similarity.
|
|
| 12. |
- Nisini, B., et al.
(författare)
-
Mapping water in protostellar outflows with Herschel PACS and HIFI observations of L1448-C
- 2013
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play.Aims. Our goal is to investigate the spatial and velocity distribution of H2O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H2O emitters among the class 0 outflows.Methods. To this end, maps of the o-H2O 1(10)-1(01) and 2(12)-1(01) transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H-2 S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H2O column densities were inferred using large velocity gradient radiative transfer calculations.Results. The water distribution appears to be clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557 GHz line is confined to radial velocities in the range +/- 10-50 km s(-1), but extended emission at extreme velocities (up to v(r) similar to 80 km s(-1)) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H2O 1(10)-1(01)/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H2O lines and the comparison with H-2 indicate averaged T-kin and n(H-2) values of similar to 300-500 K and 5 x 10(6) cm(-3), respectively, while a water abundance with respect to H-2 of about 0.5-1x10(-6) along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H2O chemistry.Conclusions. The results of our analysis show that the bulk of the observed H2O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H2O gas in the post-shock region. Among the possible scenarios, we favor H2O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength.
|
|
| 13. |
- Santangelo, G., et al.
(författare)
-
First spectrally-resolved H-2 observations towards HH 54 Low H2O abundance in shocks
- 2014
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 569, s. Art. no. L8-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Herschel observations suggest that the H2O distribution in outflows from low-mass stars resembles the H-2 emission. It is still unclear which of the different excitation components that characterise the mid-and near-IR H-2 distribution is associated with H2O. Aims. The aim is to spectrally resolve the different excitation components observed in the H-2 emission. This will allow us to identify the H-2 counterpart associated with H2O and finally derive directly an H2O abundance estimate with respect to H-2. Methods. We present new high spectral resolution observations of H-2 0-0 S(4), 0-0 S(9), and 1-0 S(1) towards HH 54, a bright nearby shock region in the southern sky. In addition, new Herschel/HIFI H2O (2(12)-1(01)) observations at 1670 GHz are presented. Results. Our observations show for the first time a clear separation in velocity of the different H-2 lines: the 0-0 S(4) line at the lowest excitation peaks at -7 kms(-1), while the more excited 0-0 S(9) and 1-0 S(1) lines peak at -15 km s(-1). H2O and high-J CO appear to be associated with the H-2 0-0 S(4) emission, which traces a gas component with a temperature of 700-1000 K. The H2O abundance with respect to H-2 0-0 S(4) is estimated to be X(H2O)
|
|
| 14. |
- Santangelo, G., et al.
(författare)
-
Water distribution in shocked regions of the NGC 1333-IRAS 4A protostellar outflow
- 2014
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 568, s. Article no. A125-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Water is a key molecule in protostellar environments because its line emission is very sensitive to both the chemistry and the physical conditions of the gas. Observations of H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observatory have highlighted the complexity of H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ line profiles, in which different kinematic components can be distinguished. Aims. The goal is to study the spatial distribution of H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ, in particular of the different kinematic components detected in H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ emission, at two bright shocked regions along IRAS 4A, one of the strongest H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ emitters among the Class 0 outflows. Methods. We obtained Herschel-PACS maps of the IRAS 4A outflow and HIFI observations of two shocked positions. The largest HIFI beam of 38'' at 557 GHz was mapped in several key water lines with different upper energy levels, to reveal possible spatial variations of the line profiles. A large velocity gradient (LVG) analysis was performed to determine the excitation conditions of the gas. Results. We detect four H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ lines and CO (16-15) at the two selected shocked positions. In addition, transitions from related outflow and envelope tracers are detected. Different gas components associated with the shock are identified in the H2O emission. In particular, at the head of the red lobe of the outflow, two distinct gas components with different excitation conditions are distinguished in the HIFI emission maps: a compact component, detected in the ground-state water lines, and a more extended one. Assuming that these two components correspond to two different temperature components observed in previous H2O and CO studies, the LVG analysis of the H2O line emission from low-mass protostars and their associated outflows performed with HIFI onboard the Herschel Space Observ emission suggests that the compact (about 32, corresponding to about 700 AU) component is associated with a hot (T similar to 1000 K) gas with densities n(H2) similar to (1-4) x 10(5) cm(-3), whereas the extended (10 ''-17 '', corresponding to 2400-4000 AU) one traces a warm (T similar to 300-500 K) and dense gas (n(H2) similar to (3-5) x 10(7) cm(-3)). Finally, using the CO (16-15) emission observed at R2 and assuming a typical CO/H-2 abundance of 10(-4), we estimate the H2O/H-2 abundance of the warm and hot components to be (7-10) x 10(-7) and (3-7) x 10(-5). Conclusions. Our data allowed us, for the first time, to resolve spatially the two temperature components previously observed with HIFI and PACS. We propose that the compact hot component may be associated with the jet that impacts the surrounding material, whereas the warm, dense, and extended component originates from the compression of the ambient gas by the propagating flow.
|
|
| 15. |
- Santangelo, G., et al.
(författare)
-
The Herschel HIFI water line survey in the low-mass proto-stellar outflow L1448
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. As part of the WISH (Water In Star-forming regions with Herschel) key project, systematic observations of H2O transitions in young outflows are being carried out with the aim of understanding the role of water in shock chemistry and its physical and dynamical properties. We report on the observations of several ortho-and para-H2O lines performed with the HIFI instrument toward two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H2O emission and to infer any dependence on velocity. Methods. We used a large velocity gradient (LVG) analysis to derive the main physical parameters of the emitting regions, namely n(H-2), T-kin, N(H2O) and emitting-region size. We compared these with other main shock tracers, such as CO, SiO and H-2 and with shock models available in the literature. Results. These observations provide evidence that the observed water lines probe a warm (T-kin similar to 400-600 K) and very dense (n similar to 10(6)-10(7) cm(-3)) gas that is not traced by other molecules, such as low-J CO and SiO, but is traced by mid-IR H-2 emission. In particular, H2O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, of which the component at higher velocity (R4-HV) is less extended and less dense than the low velocity component (R4-LV). H2O column densities of about 2 x 10(13) and 4 x 10(14) cm(-2) were derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H2O column density in the range 10(14)-5 x 10(14) cm(-2), corresponding to H2O/H-2 abundances in the range 0.5-1 x 10(-5). The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low-velocity J-type shock with strong compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce the whole characteristics of the observed emission.
|
|
| 16. |
- Vasta, M., et al.
(författare)
-
Water emission from the chemically rich outflow L1157
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537, s. Article Number: A98 -
-
Tidskriftsartikel (refereegranskat)abstract
- Context. In the framework of the Herschel-WISH key program, several ortho-H2O and para-H2O emission lines, in the frequency range from 500 to 1700 GHz, were observed with the HIFI instrument in two bow-shock regions (B2 and R) of the L1157 cloud, which hosts what is considered to be the prototypical chemically-rich outflow. Aims. Our primary aim is to analyse water emission lines as a diagnostic of the physical conditions in the blue (B2) and red-shifted (R) lobes to compare the excitation conditions. Methods. For this purpose, we ran the non-LTE RADEX model for a plane-parallel geometry to constrain the physical parameters (T-kin, N-H2O and nH(2)) of the water emission lines detected. Results. A total of 5 ortho- and para-(H2O)-O-16 plus one o-(H2O)-O-18 transitions were observed in B2 and R with a wide range of excitation energies (27K = 300 K). The presence of the broad red-shifted wings and multiple peaks in the spectra of the R region, prompted the modelling of two components. High velocities are associated with relatively low temperatures (similar to 100 K), N-H2O similar or equal to 5 x 10(12)-5 x 10(13) cm(-2) and densities nH(2) similar or equal to 10(6)-10(8) cm(-3). Lower velocities are associated with higher excitation conditions with T-kin >= 300 K, very dense gas (nH(2) similar to 10(8) cm(-3)) and low column density (N-H2O = 15 '') region, whilst we cannot rule out the possibility that the emission in R arises from a smaller (>3 '') region. In this context, H2O seems to be important in tracing different gas components with respect to other molecules, e.g. such as SiO, a classical jet tracer. We compare a grid of C-and J-type shocks spanning different velocities (10 to 40 km s(-1)) and two pre-shock densities (2 x 10(4) and 2 x 10(5) cm(-3)), with the observed intensities. Although none of these models seem to be able to reproduce the absolute intensities of the water emissions observed, it appears that the occurrence of J-shocks, which can compress the gas to very high densities, cannot be ruled out in these environments.
|
|
