arXiv subjects

Dirk Froebrich

Publications and source records attributed to Dirk Froebrich.

41 records · Page 3Linked to original sources

arXiv2005

Constraining the properties of spots on Pleiades very low mass stars

We present results of a multi-filter monitoring campaign for very low mass (VLM) stars in the Pleiades. Simultaneous to our I-band time series (Scholz & Eisloeffel 2004), which delivered photometric periods for nine VLM stars, we obtained light curves in the J- and H-band. One VLM star with M~0.15 Ms (BPL129) shows a period in all three wavelength bands. The amplitudes in I, J, and H are 0.035, 0.035, and 0.032 mag. These values are compared to simulations, in which we compute the photometric amplitude as a function of spot temperature and filling factor. The best agreement between observations and models is found for cool spots with a temperature contrast of 18-31% and a very low surface filling factor of 4-5%. We suggest that compared to more massive stars VLM objects may have either very few spots or a rather symmetric spot distribution. This difference might be explained with a change from a shell to a distributed dynamo in the VLM regime.

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arXiv2005

The near-infrared excitation of the HH211 protostellar outflow

The protostellar outflow HH211 is of considerable interest since it is extremely young and highly collimated. Here, we explore the outflow through imaging and spectroscopy in the near-infrared to determine if there are further diagnostic signatures of youth. We confirm the detection of a near-infrared continuum of unknown origin. We propose that it is emitted by the driving millimeter source, escapes the core through tunnels, and illuminates features aligning the outflow. Narrow-band flux measurements of these features contain an unusually large amount of continuum emission. [Fe II] emission has been detected and is restricted to isolated condensations. However, the characteristics of vibrational excitation of molecular hydrogen resemble those of older molecular outflows. We attempt to model the ordered structure of the western outflow as a series of shocks, finding that bow shocks with J-type dissociative apices and C-type flanks are consistent. Moreover, essentially the same conditions are predicted for all three bows except for a systematic reduction in speed and density with distance from the driving source. We find increased K-band extinctions in the bright regions as high as 2.9 magnitudes and suggest that the bow shocks become visible where the outflow impacts on dense clumps of cloud material. We propose that the outflow is carved out by episodes of pulsating jets. The jets, driven by central explosive events, are responsible for excavating a central tunnel through which radiation temporarily penetrates.

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arXiv2004

Star formation in globules in IC1396

We present a large-scale study of the IC1396 region using new deep NIR and optical images, complemented by 2MASS data. For ten globules in IC1396 we determine (H-K, J-H) colour-colour diagrams and identify the young stellar population. Five of these globules contain a rich population of reddened objects, most of them probably young stellar objects. Two new HH objects (HH865 and HH864) could be identified by means of [SII] emission, one of them a parsec-scale flow. Using star counts based on 2MASS data we create an extinction map of the whole region. This map is used to identify 25 globules and to estimate their mass. The globule masses show a significant increase with the distance from the exciting O6.5V star HD206267. We explain this correlation by the enhanced radiation pressure close to this star, leading to evaporation of the nearby clouds and hence smaller globule masses. We see evidence that the radiation from HD206267 has a major impact on the star formation activity in these globules.

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arXiv2004

Submillimetre continuum emission from Class 0 sources: Theory, Observations, and Modelling

We report on a study of the thermal dust emission of the circumstellar envelopes of a sample of Class 0 sources. The physical structure (geometry, radial intensity profile, spatial temperature and spectral energy distribution) and properties (mass, size, bolometric luminosity (L_bol) and temperature (T_ bol), and age) of Class 0 sources are derived here in an evolutionary context. This is done by combining SCUBA imaging at 450 and 850 microm of the thermal dust emission of envelopes of Class 0 sources in the Perseus and Orion molecular cloud complexes with a model of the envelope, with the implementation of techniques like the blackbody fitting and radiative transfer calculations of dusty envelopes, and with the Smith evolutionary model for protostars. The modelling results obtained here confirm the validity of a simple spherical symmetric model envelope, and the assumptions about density and dust distributions following the standard envelope model. The spherically model reproduces reasonably well the observed SEDs and the radial profiles of the sources. The implications of the derived properties for protostellar evolution are illustrated by analysis of the L_bol, the T_bol, and the power-law index p of the density distribution for a sample of Class 0 sources.

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arXiv2004

Which are the youngest protostars? Determining properties of confirmed and candidate Class0 sources by broad-band photometry

We searched the literature to obtain a complete list of known Class0 sources. A list of 95 confirmed or candidate objects was compiled. To the best of our knowledge, all published broad-band observations from 1mu to 3.5mm have been collected and are assembled in a catalogue. These data were used to determine physical properties (T_bol, L_bol, L_smm/L_bol, M_env) and for a uniform classification. 50 sources possess sufficient observational data and are classified as Class0 or Class0/1 objects. The source properties are compared with different evolutionary models to infer ages and masses, and their correlations are investigated. About 25% of the sources are found to be in a quiet accretion phase or possess a significantly different time evolution of the accretion rate than the average. In Taurus, with its isolated star formation mode, this seems especially to be the case.

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