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  • 1. Dahlgren, H,
    et al.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Filamentary structures in planetary nebulae2007In: Astrophysics & Space Science, Vol. 310, p. 65-Article in journal (Refereed)
  • 2. de Val-Borro, M.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Stempels, H. C.
    Peplinski, Adam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Modelling circumbinary gas flows in close T Tauri binaries2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, no 4, p. 2679-2688Article in journal (Refereed)
    Abstract [en]

    Young close binaries open central gaps in the surrounding circumbinary accretion disc, but the stellar components may still gain mass from gas crossing through the gap. It is not well understood how this process operates and how the stellar components are affected by such inflows. Our main goal is to investigate how gas accretion takes place and evolves in close T Tauri binary systems. In particular, we model the accretion flows around two close T Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission lines, although their orbital characteristics are very different. In order to derive the density and velocity maps of the circumbinary material, we employ two-dimensional hydrodynamic simulations with a locally isothermal equation of state. The flow patterns become quasi-stable after a few orbits in the frame corotating with the system. Gas flows across the circumbinary gap through the corotating Lagrangian points, and local circumstellar discs develop around both components. Spiral density patterns develop in the circumbinary disc that transport angular momentum efficiently. Mass is preferentially channelled towards the primary and its circumstellar disc is more massive than the disc around the secondary. We also compare the derived density distribution to observed line profile variability. The line profile variability tracing the gas flows in the central cavity shows clear similarities with the corresponding observed line profile variability in V4046 Sgr, but only when the local circumstellar disc emission was excluded. Closer to the stars normal magnetospheric accretion may dominate, while further out the dynamic accretion process outlined here dominates. Periodic changes in the accretion rates on to the stars can explain the outbursts of line emission observed in eccentric systems such as DQ Tau.

  • 3.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Anita Sundman2010In: Populär Astronomi, ISSN 1650-7177, no 2, p. 21-21Article in journal (Other (popular science, discussion, etc.))
  • 4.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Elefantsnablar, Tårar, Ägg och Globuletter: Nebulosornas skötebarn2010In: Populär Astronomi, ISSN 1650-7177, no 3, p. 32-36Article in journal (Other (popular science, discussion, etc.))
  • 5.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Enhanced Activity in Close T Tauri Binaries2006In: Astrophysics and Space Science, ISSN 0004-640X, Vol. 304, no 1-4, p. 149-152Article in journal (Refereed)
    Abstract [en]

    The number of confirmed and suspected close T Tauri binaries (period days) is increasing. We discuss some systems with enhanced emission line activity and periodic line profile changes. Non-axisymmetric flows of plasma in the region between the circumbinary disk and the stars can be generated through the influence of the secondary component. Such enhanced activity is found around binaries with eccentric as well as circular orbits. We discuss our observations of the T Tauri stars RW Aurigae A and RU Lupi, which may host very close brown dwarf companions. Model simulations indicate that non-axisymmetric flows are generated around close binaries with circumbinary disks, also in systems with circular orbits.

  • 6.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Får jorden en framsida och baksida ?2007In: Forskning och Framsteg, Vol. 4, no 67Article in journal (Other (popular science, discussion, etc.))
  • 7.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Globulettes as seeds of free-floating planetary-mass objects2007In: Astronomical Journal, Vol. 133, p. 1795-Article in journal (Refereed)
  • 8.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Skolornas astronomiår2009In: Populär Astronomi, Vol. 4/2009, p. 50-51Article in journal (Other (popular science, discussion, etc.))
  • 9.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    "Utblick - tillbakablick" till Göteborg2010In: Aurora, ISSN 1101-1718, Vol. 34, no 4, p. 20-23Article in journal (Other (popular science, discussion, etc.))
  • 10.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Vintergatans ensamma planeter2014In: Forskning och Framsteg, ISSN 0015-7937, no 6, p. 44-49Article in journal (Other (popular science, discussion, etc.))
  • 11.
    Gahm, Gösta F.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Petrov, P. P.
    Tambovsteva, L. V.
    Grinin, V. P.
    Stempels, H. C.
    Walter, F. M.
    S Coronae Australis: a T Tauri twin2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A117Article in journal (Refereed)
    Abstract [en]

    Context. The star S CrA is a tight visual binary consisting of two classical T Tauri stars. Both components are outstanding regarding their spectral characteristics and brightness variations. Aims. Our aim is to explore the extraordinary spectral features seen in these stars, derive stellar parameters, define spectral signatures of accreting gas and winds, estimate the inclinations of the disks, and to match numerical models with observed properties. Methods. High-resolution spectra were collected of each component over several nights at the European Southern Observatory (ESO) combined with photometric observations covering several years in UBVRI with the SMARTS telescope. The models developed include magnetospheric accretion and a disk wind. Results. Both stars undergo large variation in brightness, >= 2 mag in V band. The variations are caused mainly by variable foreground extinction from small-size dust grains, which may be carried along with the accreting gas. The photospheric absorption lines are washed out by superimposed continuous and line emission, and this veiling becomes occasionally exceptionally high. Nevertheless, we extracted the stellar spectra and found that both stars are very similar with regard to stellar parameters (T-eff, log g, v sin i, mass, radius, luminosity). The rotational periods, inferred from velocity shifts in lines originating in surface areas off-set from the pole, are also similar. Combined with the v sin i:s related inclinations were obtained, which agree well with those derived from our model simulations of Balmer line profiles: similar to 65 degrees for both stars. At this orientation the trajectories of infalling gas just above the stellar surfaces are parallel to the line-of-sight, and accordingly we observe extended red-shifted absorption components extending to +380 km s(-1), the estimated free-fall velocity at the surface. Rates of accretion and mass loss were obtained from the models. Conclusions. The two stars are remarkably similar, and S CrA can be regarded as a T Tauri twin. The components differ, however, in terms of degree of veiling and emission line profiles. We have found a good match between observed signatures of accreting gas, wind features, and rotational velocities with those resulting from our modelling for inclinations of similar to 65 degrees. These inclinations differ from those derived from interferometric near-infrared (NIR) spectroscopy, and possible causes for this puzzling discrepancy are discussed.

  • 12.
    Gahm, Gösta F.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Walter, F. M.
    Stempels, H. C.
    Petrov, P. P.
    Herczeg, G. J.
    Unveiling extremely veiled T Tauri stars2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 482, no 3, p. L35-L38Article in journal (Refereed)
    Abstract [en]

    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.

  • 13.
    Gahm, Gösta
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kristoferson, Lars
    Jord, vatten, luft, eld2013Book (Other (popular science, discussion, etc.))
    Abstract [sv]

    Människan mäter sig nu med naturen i att förändra klimatet, livets villkor och dess utveckling. Jorden är en utsatt planet, inte bara genom människans framfart utan också genom krafter utifrån och ett bombardemang av strålning, partiklar och kroppar från rymden. Miljön formas genom en mångfald processer som verkat sedan jorden bildades för 4,6 miljarder år sedan.

    Jord, vatten, luft, eld är en pedagogisk och medryckande berättelse om vår värld, från atomerna som utgör våra kroppar till galaxerna som omger oss.

  • 14.
    Gahm, Gösta
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Persson, C. M.
    Makela, M. M.
    Haikala, L. K.
    Mass and motion of globulettes in the Rosette Nebula2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 555, p. A57-Article in journal (Refereed)
    Abstract [en]

    Context. Tiny molecular clumps are abundant in many H II regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark patches against the background of bright nebulosity. The majority of the globulettes were found to be of planetary mass in a previous optical investigation, while the largest objects may contain more than half a solar mass. Aims. We aim to clarify the physical nature of globulettes by deriving densities and masses, and to determine their velocities as a function of position over the nebula. This information will provide clues to the question of origins, evolution, and fate of globulettes. The Rosette Nebula is relatively rich in globulettes, and we selected a sample of well-confined objects of different sizes for the present investigation. Methods. Radio observations were made of molecular line emission from 16 globulettes combined with near-infrared (NIR) broad-band JHKs and narrow-band Paschen beta and H-2 imaging. Ten objects, for which we collected information from several transitions in (CO)-C-12 and (CO)-C-13, were modelled using a spherically symmetric model. Results. Practically all globulettes were detected in our CO survey. The observed (CO)-C-12 (3-2) and (2-1) line temperatures range from 0.6 K to 6 K, the (CO)-C-13 being a third of this. As a rule, the lines are narrow, similar to 1.0 km s(-1). The best fit to observed line ratios and intensities was obtained by assuming a model composed of a cool and dense centre and warm and dense surface layer. This model provides estimates of maximum and minimum mass; the average masses range from about 50 to 500 Jupiter masses, which is similar to earlier estimates based on extinction measures. The selected globulettes are dense, n(H) similar to 10(4) cm(-3), with very thin layers of fluorescent H-2 emission, showing that the gas is in molecular form just below the surface. The NIR data show that several globulettes are very opaque and contain dense cores. No infrared-excess stars in the fields are associated with globulettes. Internal gas motions are weak, but some larger objects show velocity-shifted components associated with tails. However, most globulettes show no signs of tails or pronounced bright rims in contradiction to current numerical simulations of clumps exposed to intense stellar radiation. Because of the high density encountered already at the surface, the rims become thin, as evidenced by our P beta images, which also show extended emission that most likely comes from the backside of the globulettes. We conclude that the entire complex of shells, elephant trunks, and globulettes in the northern part of the nebula is expanding with nearly the same velocity of similar to 22 km s(-1), and with a very small spread in velocity among the globulettes. We note that the velocities observed for background shells do not fit into a spherically expanding nebular complex. Conclusions. Some globulettes are in the process of detaching from elephant trunks and shells, while other more isolated objects must have detached long ago and are lagging behind in the general expansion of the molecular shell. Several globulettes are presently subject to heavy erosion from the intense radiation field from the central stars and eject gas streams (tails), while other quite isolated objects lack such signatures. We envision that after detachment, the objects erode to isolated and dense clumps. The suggestion that some globulettes might collapse to form planetary-mass objects or brown dwarfs is strengthened by our finding of dense cores in several objects. Such free-floating low-mass objects would move at high speed from the start and escape from the region.

  • 15. Grenman, T.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    The tiny globulettes in the Carina nebula2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, p. A107-Article in journal (Refereed)
    Abstract [en]

    Context. Small molecular cloudlets are abundant in many H II regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark silhouettes against the bright nebular background. Aims. We aim to make an inventory of the population of globulettes in the Carina nebula complex, and to derive sizes and masses for comparisons with similar objects found in other H II regions. Methods. The globulettes were identified from H alpha images collected at the Hubble Space Telescope. Results. We have located close to 300 globulettes in the Carina complex, more than in any other region surveyed so far. The objects appear as well-confined dense clumps and, as a rule, lack thinner envelopes and tails. Objects with bright rims are in the minority, but more abundant than in other regions surveyed. Some globulettes are slightly elongated with their major axes oriented in the direction of young clusters in the complex. Many objects are quite isolated and reside at projected distances >1.5 pc from other molecular structures in the neighbourhood. No globulette coincides in position with recognized pre-main-sequence objects in the area. The objects are systematically much smaller, less massive, and much denser than those surveyed in other H II regions. Practically all globulettes are of planetary mass, and most have masses less than one Jupiter mass. The average number densities exceed 105 cm 3 in several objects. We have found a statistical relation between density and radius (mass) in the sense that the smallest objects are also the densest. Conclusions. The population of small globulettes in Carina appears to represent a more advanced evolutionary state than those investigated in other H II regions. The objects are subject to erosion in the intense radiation field, which would lead to a removal of any thinner envelope and an unveiling of the core, which becomes more compact with time. We discuss the possibility that the core may become gravitationally unstable, in which case free-floating planetary mass objects can form.

  • 16. Grenman, T.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Elfgren, E.
    Dusty globules in the Crab Nebula2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A110Article in journal (Refereed)
    Abstract [en]

    Context. Dust grains are widespread in the Crab Nebula. A number of small, dusty globules, are visible as dark spots against the background of continuous synchrotron emission in optical images. Aims. Our aim is to catalogue such dusty globules and investigate their properties. Methods. From existing broad-band images obtained with the Hubble Space Telescope, we located 92 globules, for which we derived positions, dimensions, orientations, extinctions, masses, proper motions, and their distributions. Results. The globules have mean radii ranging from 400 to 2000 AU and are not resolved in current infrared images of the nebula. The extinction law for dust grains in these globules matches a normal interstellar extinction law. Derived masses of dust range from 1 to 60 x 10(-6) M-circle dot, and the total mass contained in globules constitute a fraction of approximately 2% or less of the total dust content of the nebula. The globules are spread over the outer part of the nebula, and a fraction of them coincide in position with emission filaments, where we find elongated globules that are aligned with these filaments. Only 10% of the globules are coincident in position with the numerous H-2-emitting knots found in previous studies. All globules move outwards from the centre with transversal velocities of 60 to 1600 km s(-1), along with the general expansion of the remnant. We discuss various hypotheses for the formation of globules in the Crab Nebula.

  • 17. Grinin, V.
    et al.
    Stempels, H. C.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sergeev, S.
    Arkharov, A.
    Barsunova, O.
    Tambovtseva, L.
    The unusual pre-main-sequence star V718 Per: Photometry and spectroscopy across the eclipse2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 489, no 3, p. 1233-1238Article in journal (Refereed)
  • 18. Haikala, L. K.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Grenman, T.
    Maekelae, M. M.
    Persson, C. M.
    Radio observations of globulettes in the Carina nebula2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 602, article id A61Article in journal (Refereed)
    Abstract [en]

    Context. The Carina nebula hosts a large number of globulettes. An optical study of these tiny molecular clouds shows that the majority are of planetary mass, but there are also those with masses of several tens up to a few hundred Jupiter masses. Aims. We seek to search for, and hopefully detect, molecular line emission from some of the more massive objects; in case of successful detection we aim to map their motion in the Carina nebula complex and derive certain physical properties. Methods. We carried out radio observations of molecular line emission in (CO)-C-12 and (CO)-C-13 (2-1) and (3-2) of 12 globulettes in addition to positions in adjacent shell structures using APEX. Results. All selected objects were detected with radial velocities shifted relative to the emission from related shell structures and background molecular clouds. Globulettes along the western part of an extended dust shell show a small spread in velocity with small velocity shifts relative to the shell. This system of globulettes and shell structures in the foreground of the bright nebulosity surrounding the cluster Trumpler 14 is expanding with a few km s(-1) relative to the cluster. A couple of isolated globulettes in the area move at similar speed. Compared to similar studies of the molecular line emission from globulettes in the Rosette nebula, we find that the integrated line intensity ratios and line widths are very different. The results show that the Carina objects have a different density/temperature structure than those in the Rosette nebula. In comparison the apparent size of the Carina globulettes is smaller, owing to the larger distance, and the corresponding beam filling factors are small. For this reason we were unable to carry out a more detailed modelling of the structure of the Carina objects in the way as performed for the Rosette objects. Conclusions. The Carina globulettes observed are compact and denser than objects of similar mass in the Rosette nebula. The distribution and velocities of these globulettes suggest that they have originated from eroding shells and elephant trunks. Some globulettes in the Trumpler 14 region are quite isolated and located far from any shell structures. These objects move at a similar speed as the globulettes along the shell, suggesting that they once formed from cloud fragments related to the same foreground shell.

  • 19.
    Larsson, B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    First detection of molecular oxygen in the interstellar medium - O2 observed with Odin in the rho Ophiuch cloud2007In: Astronomy and Astrophysics, Vol. 466, p. 999-Article in journal (Other (popular science, discussion, etc.))
  • 20.
    Larsson, Bengt
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, Rene
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pagani, Laurent
    Bergman, Per
    Bernath, Peter
    Biver, Nicolas
    Black, John
    Booth, Roy
    Buat, Veronique
    Crovisier, Jacques
    Curry, Charles
    Dahlgren, Magnus
    Encrenaz, Pierre
    Falgarone, Edith
    Feldman, Paul
    Fish, Michel
    Florén, Hans-Gustav
    Stockholm University, Faculty of Science, Department of Astronomy.
    Fredrixon,
    Frisk, Urban
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gerin, Maryvonne
    Hagström, Magne
    Harju, Jorma
    Hasegawa, Tatsuhiko
    Hjalmarsson, Åke
    Johansson, Lars
    Justtanout, Kay
    Stockholm University, Faculty of Science, Department of Astronomy.
    Klotz, Alain
    Kytölä, Erikii
    Kwok, Sun
    Lecacheux, Alain
    Liljeström, Tarja
    Llewellyn, Edward
    Lundin, Stefan
    Mégie, Gérard
    Mitchell, Gary
    Murtagh, Donal
    Nordh, Lennart
    Nyman, Lars-Åke
    Olberg, Michael
    Olofsson, Henrik
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Hans
    Stockholm University, Faculty of Science, Department of Astronomy.
    Persson, Glen
    Plume, Rene
    Rickman, Hans
    Ristorcelli, Isabelle
    Rydbeck, Gustaf
    Sandqvist, Aage
    Stockholm University, Faculty of Science, Department of Astronomy.
    von Scheele, Fredrik
    Serra, Guy
    Torchinsky, Steve
    Tothill, Nick
    Volk, Kevin
    Wiklind, Tommy
    Wilson, Christine
    Winnberg, Anders
    Witt, George
    Department of Meteorology.
    Molecular oxygen in the rho Ophiuchi cloud2007In: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 466, no 3, p. 5-Article in journal (Refereed)
    Abstract [en]

    Context: Molecular oxygen, O2, has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission.

    Aims: The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods: The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs.

    Results: We detect a spectral line at v_LSR =+3.5 km s-1 with Δ v_FWHM=1.5 km s-1, parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O2 (NJ = 11 - 1_0) transition at 118 750.343 MHz.

    Conclusions: The abundance of molecular oxygen, relative to H{2} , is 5 × 10-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

    Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Étude Spatiale (CNES). The Swedish Space Corporation has been the industrial prime contractor and also is operating the satellite. Appendix A is only available in electronic form at http://www.aanda.org

  • 21. Makela, M. M.
    et al.
    Haikala, L. K.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Rosette globulettes and shells in the infrared2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, p. A108-Article in journal (Refereed)
    Abstract [en]

    Context. Giant galactic H II regions surrounding central young clusters show compressed molecular shells, which have broken up into clumps, filaments, and elephant trunks interacting with UV light from central OB stars. Tiny, dense clumps of subsolar mass, called globulettes, form in this environment. Aims. We observe and explore the nature and origin of the infrared emission and extinction in these cool, dusty shell features and globulettes in one H II region, the Rosette nebula, and search for associated newborn stars. Methods. We imaged the northwestern quadrant of the Rosette nebula in the near-infrared (NIR) through wideband JHKs filters and narrowband H-2 1-0 S(1) and P beta plus continuum filters using the Son of Isaac (SOFI) instrument at the New Technology Telescope (NTT) at European Southern Observatory (ESO). We used the NIR images to study the surface brightness of the globulettes and associated bright rims. We used the NIR JHKs photometry to create a visual extinction map and to search for objects with NIR excess emission. In addition, archival images from Spitzer Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) 24 mu m and Herschel Photoconductor Array Camera and Spectrometer (PACS) observations, covering several bands in the mid-infrared and far-infrared, were used to further analyze the stellar population, to examine the structure of the trunks and other shell structures and to study this Rosette nebula photon-dominated region in more detail. Results. The globulettes and elephant trunks have bright rims in the Ks band, which are unresolved in our images, on the sides facing the central cluster. An analysis of 21 globulettes, where surface brightness in the H-2 1-0 S(1) line at 2.12 mu m is detected, shows that approximately a third of the surface brightness observed in the Ks filter is due to this line: the observed average of the H-2/Ks surface brightness is 0.26 +/- 0.02 in the globulettes' cores and 0.30 +/- 0.01 in the rims. The estimated H-2 1-0 S(1) surface brightness of the rims is -3-8 x 10(-8) Wm(-2) sr(-1) mu m(-1). The ratio of the surface brightnesses support fluorescence instead of shocks as the H-2 excitation mechanism. The globulettes have number densities of n(H-2) similar to 10(-4) cm(-3) or higher. We estimated masses of individual globulettes and compared them to the results from previous optical and radio molecular line surveys. We confirm that the larger globulettes contain very dense cores, that the density is also high farther out from the core, and that their mass is subsolar. Two NIR protostellar objects were found in an elephant trunk and one was found in the most massive globulette in our study.

  • 22. Mäkelä, M. M.
    et al.
    Haikala, L. K.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Rosette nebula globules: Seahorse giving birth to a star2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A82Article in journal (Refereed)
    Abstract [en]

    Context. The Rosette nebula is an H II region ionized mainly by the stellar cluster NGC 2244. Elephant trunks, globules, and globulettes are seen at the interface where the H II region and the surrounding molecular shell meet.

    Aims. We have observed a field in the northwestern part of the Rosette nebula where we study the small globules protruding from the shell. Our aim is to measure their properties and study their star-formation history in continuation of our earlier study of the features of the region.

    Methods. We imaged the region in broadband near-infrared (NIR) JsHKs filters and narrowband H-2 1-0 S(1), P beta, and continuum filters using the SOFI camera at the ESO/NTT. The imaging was used to study the stellar population and surface brightness, create visual extinction maps, and locate star formation. Mid-infrared (MIR) Spitzer IRAC and WISE and optical NOT images were used to further study the star formation and the structure of the globules. The NIR and MIR observations indicate an outflow, which is confirmed with CO observations made with APEX.

    Results. The globules have mean number densities of similar to 4.6x10(4) cm(-3). P beta is seen in absorption in the cores of the globules where we measure visual extinctions of 11-16 mag. The shell and the globules have bright rims in the observed bands. In the Ks band 20 to 40% of the emission is due to fluorescent emission in the 2.12 mu m H-2 line similar to the tiny dense globulettes we studied earlier in a nearby region. We identify several stellar NIR excess candidates and four of them are also detected in the Spitzer IRAC 8.0 mu m image and studied further. We find an outflow with a cavity wall bright in the 2.124 mu m H-2 line and at 8.0 mu m in one of the globules. The outflow originates from a Class I young stellar object (YSO) embedded deep inside the globule. An H alpha image suggests the YSO drives a possible parsec-scale outflow. Despite the morphology of the globule, the outflow does not seem to run inside the dusty fingers extending from the main globule body.

  • 23.
    Petrov, P.
    et al.
    Crimean Astrophysical Observatory.
    Artemenko, A,
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Stempels, E.
    Walter, F.
    Cromospheres in classical T Tauri stars2011In: Nordic Optical Telescope, Annual Report 2010, p. 16-17Article in journal (Other academic)
  • 24. Petrov, P. P.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Djupvik, A. A.
    Babina, E. V.
    Artemenko, S. A.
    Grankin, K. N.
    Another deep dimming of the classical T Tauri star RW Aurigae A2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 577, article id A73Article in journal (Refereed)
    Abstract [en]

    Context. RWAur A is a classical T Tauri star (CTTS) with an unusually rich emission line spectrum. In 2014 the star faded by similar to 3 mag in the V band and went into a long-lasting minimum. In 2010 the star underwent a similar fading, although less pronounced. These events in RW Aur A are very unusual among the CTTS, and have been attributed to occultations by passing dust clouds. Aims. We want to find out if any spectral changes took place after the last fading of RW Aur A with the intention of gathering more information on the occulting body and the cause of the phenomenon. Methods. We collected spectra of the two components of RW Aur. The photometry was performed before and during the minimum. Results. The overall spectral signatures reflecting emission from accretion flows from disk to star did not change after the fading. However, blue-shifted absorption components related to the stellar wind increased in strength in certain resonance lines, and the profiles and strengths but not the fluxes of forbidden lines became drastically different. Conclusions. The extinction through the obscuring cloud is grey indicating the presence of large dust grains. At the same time, there are no traces of related absorbing gas. The cloud occults the star and the interior part of the stellar wind, but not the wind or jet further out. The dimming in 2014 was not accompanied by changes in the accretion flows at the stellar surface. There is evidence that the structure and velocity pattern of the stellar wind did change significantly. The dimmings could be related to passing condensations in a tidally disrupted disk, as proposed earlier, but we also speculate that large dust grains have been stirred up from the inclined disk into the line of sight through the interaction with an enhanced wind.

  • 25. Petrov, P. P.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Herczeg, G. J.
    Stempels, H. C.
    Walter, F. M.
    Doppler probe of accretion onto a T Tauri star2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 568, p. L10-Article in journal (Refereed)
    Abstract [en]

    Context. The YY Ori stars are T Tauri stars with prominent time-variable redshifted absorption components that flank certain emission lines. S CrA, one of the brightest of these stars, affords the rare opportunity of directly probing the accretion processes on the line of sight to one of the components of this wide visual pair. Aims. We followed the spectral changes in S CrA to derive the physical structure of the accreting gas. Methods. A series of high-resolution spectra of the two components of S CrA was obtained during four nights with the UVES spectrograph at the Very Large Telescope. Results. We found that both stars are very similar with regard to surface temperature, radius, and mass. Variable redshifted absorption components are particularly prominent in the SE component. During one night, this star developed a spectrum unique among the T Tauri stars: extremely strong and broad redshifted absorption components appeared in many lines of neutral and ionized metals, in addition to those of hydrogen and helium. The absorption depths of cooler, low-ionization lines peak at low velocities - while more highly ionized lines have peak absorption depths at high velocities. The different line profiles indicate that the temperature and density of the accretion stream increase as material approaches the star. We derive the physical conditions of the flow at several points along the accretion funnel directly from the spectrum of the infalling gas. We estimated mass accretion rates of about 10(-7) M-circle dot/yr, which is similar to that derived from the relation based on the strength of Ha emission line. Conclusions. This is the first time the density and temperature distributions in accretion flows around a T Tauri star have been inferred from observations. Compared with predictions from standard models of accretion in T Tauri stars, which assume a dipole stellar magnetic field, we obtained higher densities and a steeper temperature rise toward the star.

  • 26. Petrov, P. P.
    et al.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Stempels, H. C.
    Walter, F. M.
    Artemenko, S. A.
    Accretion-powered chromospheres in classical T Tauri stars2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 535, p. A6-Article in journal (Refereed)
    Abstract [en]

    Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion. From high-resolution spectroscopy of DR Tauri we found that the amount of veiling in this star varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. This effect can be shown to exist in several other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.

  • 27. Petrov, P. P.
    et al.
    Grankin, K. N.
    Gameiro, J. F.
    Artemenko, S. A.
    Babina, E. V.
    de Albuquerque, R. M. G.
    Djupvik, A. A.
    Gahm, Gösta F.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Shenavrin, V. I.
    Irsmambetova, T. R.
    Fernandez, M.
    Mkrtichian, D. E.
    Gorda, S. Yu.
    Dynamics of wind and the dusty environments in the accreting T Tauri stars RY Tauri and SU Aurigae2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 483, no 1, p. 132-146Article in journal (Refereed)
    Abstract [en]

    Classical T Tauri stars with ages of less than 10 Myr possess accretion discs. Magnetohydrodynamic processes at the boundary between the disc and the stellar magnetosphere control the accretion and ejections gas flows. We carried out a long series of simultaneous spectroscopic and photometric observations of the classical T Tauri stars, RY Tauri and SU Aurigae, with the aim to quantify the accretion and outflow dynamics at time-scales from days to years. It is shown that dust in the disc wind is the main source of photometric variability of these stars. In RY Tau, we observed a new effect: during events of enhanced outflow, the circumstellar extinction becomes lower. The characteristic time of changes in outflow velocity and stellar brightness indicates that the obscuring dust is near the star. The outflow activity in both stars is changing on a time-scale of years. Periods of quiescence in the variability of the H alpha profile were observed during the 2015-2016 period in RY Tau and during the 2016-2017 period in SU Aur. We interpret these findings in the framework of the magnetospheric accretion model, and we discuss how the global stellar magnetic field can influence the long-term variations of the outflow activity.

  • 28. Stempels, H. C.
    et al.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Periodic radial velocity variations in RU Lupi2007In: Astronomy & Astrophysics, Vol. 461, p. 253-Article in journal (Refereed)
  • 29. Stempels, H. C.
    et al.
    Gahm, Gösta
    Stockholm University, Faculty of Science, Department of Astronomy.
    Petrov, Petr
    Periodic radial velocity variations in RU Lupi2007In: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 461, no 1, p. 253-259Article in journal (Refereed)
    Abstract [en]

    Context: . RU Lup is a Classical T Tauri star with unusually strong emission lines, which has been interpreted as manifestations of accretion. Recently, evidence has accumulated that this star might have a variable radial velocity.

    Aims: .We intended to investigate in more detail the possible variability in radial velocity using a set of 68 high-resolution spectra taken at the VLT (UVES), the AAT (UCLES) and the CTIO (echelle).

    Methods: .Using standard cross-correlation techniques, we determined the radial velocity of RU Lup. We analysed these results with Phase-dispersion minimization and the Lomb-Scargle periodogram and searched for possible periodicities in the obtained radial velocities. We also analysed changes in the absorption line shapes and the photometric variability of RU Lup.

    Results: .Our analysis indicated that RU Lup exhibits variations in radial velocity with a periodicity of 3.71 days and an amplitude of 2.17 km s-1. These variations can be explained by the presence of large spots, or groups of spots, on the surface of RU Lup. We also considered a low-mass companion and stellar pulsations as alternative sources for these variations but found these to be unlikely.

1 - 29 of 29
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