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- Gahm, Gösta F., 1942-, et al.
(författare)
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Expanding shells around young clusters - S 171/Be 59
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 663
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Tidskriftsartikel (refereegranskat)abstract
- Context. Some HII regions that surround young stellar clusters are bordered by molecular shells that appear to expand at a rate inconsistent with our current model simulations. In this study we focus on the dynamics of Sharpless 171 (including NGC 7822), which surrounds the cluster Berkeley 59. Aims. We aim to compare the velocity pattern over the molecular shell with the mean radial velocity of the cluster for estimates of the expansion velocities of different shell structures, and to match the observed properties with model simulations. Methods. Optical spectra of 27 stars located in Berkeley 59 were collected at the Nordic Optical Telescope, and a number of molecular structures scattered over the entire region were mapped in 13CO(1- 0) at Onsala Space Observatory. Results. We obtained radial velocities and MK classes for the clustera's stars. At least four of the O stars are found to be spectroscopic binaries, in addition to one triplet system. From these data we obtain the mean radial velocity of the cluster. From the 13CO spectra we identify three shell structures, expanding relative to the cluster at moderate velocity (4 km s- 1), high velocity (12 km s- 1), and in between. The high-velocity cloudlets extend over a larger radius and are less massive than the low-velocity cloudlets. We performed a model simulation to understand the evolution of this complex. Conclusions. Our simulation of the Sharpless 171 complex and Berkeley 59 cluster demonstrates that the individual components can be explained as a shell driven by stellar winds from the massive cluster members. However, our relatively simple model produces a single component. Modelling of the propagation of shell fragments through a uniform interstellar medium demonstrates that dense cloudlets detached from the shell are decelerated less efficiently than the shell itself. They can reach greater distances and retain higher velocities than the shell.
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- Gahm, G. F., et al.
(författare)
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Face to phase with RU Lupi
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 560, s. A57-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Some classical T Tauri stars, with intense line and continuous excess emission, show extremely complex spectral variations. Aims. We aim to map and interpret the spectral variations in one such extreme T Tauri star, namely RU Lupi, and to explore how the changes are related to stellar brightness and rotational phase.Methods. We followed the star over three observing runs, each covering a few days, collecting high-resolution optical spectra. In connection to the third run, complementary NIR spectra, multicolour photometric data, and X-ray observations were obtained.Results. The stellar photospheric absorption line spectrum is weakened by superimposed emission, and this veiling becomes extremely high on occasion. Interpreted as a variable continuous excess emission, its contribution would amount to several times the stellar continuum brightness. However, the stellar brightness does not change much when the veiling changes, and we conclude that the veiling is dominated by narrow line emission that fills in the photospheric lines. Continuous emission, originating at the hot spot on the stellar surface, plays a dominant role only at lower degrees of veiling. The radial velocity of narrow emission components in lines of He I vary periodically in anti-phase with the stellar velocity, reflecting the location and motion of the accretion footprint. The blue-shifted wings in He I, related to a stellar wind, are remarkably stable in equivalent width. This implies that the line flux responds directly to changes in the veiling, which in turn is related to the accretion rate close to the star. In contrast, the equivalent widths of the red-shifted wings change with rotational phase. From the pattern of variability we infer that these wings originate in accreting gas close to the star, and that the accretion funnels are bent and trail the hot spot. The profiles of the forbidden lines of [O I] and [S II] are very stable in strength and shape over the entire observing period, and like a system of narrow, blue-shifted absorption features seen in lines of Ca II and Na I, they originate at larger distances from the star in the disk wind. Slightly blue-shifted emission components are present in the forbidden lines and might be related to a wide angle molecular disk wind proposed by others.
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- Gahm, G. F., et al.
(författare)
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The Threaded Molecular Clumps of Chamaeleon III
- 2002
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 389:2, s. 577-588
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Tidskriftsartikel (refereegranskat)abstract
- We have mapped large areas in the complex of molecular clouds with the SEST in 13CO( J=1-0) and in C 18O( J=1-0). The stronger CO emission coincides with areas of cold dust emission, which is distributed in long, but thin, wavy filaments. We identify some 40 clumps of enhanced CO emission in these filaments. In the southern part of the clumps are equidistant along the main zig-zag shaped filament. Here we find two systems of filaments moving at different radial velocities. At least part of the zig-zag patterns visible on optical images may be caused by overlapping filaments. All clumps are small (typically 0.02-0.05 pc in radius), and of small mass (typically 0.1-0.7 , when assuming the "standard" C 18O/H 2 column density ratio). Also the average number densities are small, cm -3, and the density contrast between clump and interclump gas is only ~10. In addition the values of are unusually small, 0.03-0.33. These clumps have smaller masses than those so far identified in other molecular clouds. Previously reported clumps of larger masses in turn out to be composed of assemblies of clumps. There are no signs of star formation in (unlike and ), and our results indicate also that such activity is not expected. However, with the velocity dispersion of 0.2 km s -1 the clumps would leave the thin filaments on short timescales, and if the clumps as such are not confined by some external force, they would also lose their identity on even shorter timescales. We discuss the possibility that the clumps are confined by electromagnetic forces, and show that this may work with reasonable assumptions on the required magnetic field strength. We also discuss the possibility that the clumps are attached to magnetic ropes along the filamentary axis, in which case the clumps could swing back and forth perpendicularly to the axis, like they were threaded on elastic strings.
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- Gahm, Gösta F., et al.
(författare)
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Unveiling extremely veiled T Tauri stars
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 482:3, s. L35-L38
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Tidskriftsartikel (refereegranskat)abstract
- Context. Photospheric absorption lines in classical T Tauri stars (CTTS) are weak compared to normal stars. This so-called veiling is normally identified with an excess continuous emission formed in shock-heated gas at the stellar surface below the accretion streams.Aims. We have selected four stars (RW Aur A, RU Lup, S CrA NW and S CrA SE) with unusually strong veiling to make a detailed investigation of veiling versus stellar brightness and emission line strengths for comparisons to standard accretion models.Methods. We have monitored the stars photometrically and spectroscopically at several epochs.Results. In standard accretion models a variable accretion rate will lead to a variable excess emission. Consequently, the stellar brightness should vary accordingly. We find that the veiling of absorption lines in these stars is strongly variable and usually so large that it would require the release of several stellar luminosities of potential energy. At states of very large line dilution, the correspondingly large veiling factors derived correlate only weakly with brightness. Moreover, the emission line strengths violate the expected trend of veiling versus line strength. The veiling can change dramatically in one night, and is not correlated with the phase of the rotation periods found for two stars.Conclusions. We show that in at least three of the stars, when the veiling becomes high, the photospheric lines become filled-in by line emission, which produces large veiling factors unrelated to changes in any continuous emission from shocked regions. We also consider to what extent extinction by dust and electron scattering in the accretion stream may affect veiling measures in CTTS. We conclude that the degree of veiling cannot be used as a measure of accretion rates in CTTS with rich emission line spectra.
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