LA VIOLENCIA DE GÉNERO EN EL CONTEXTO ESCOLAR

Using spectro-interferometric observations obtained with AMBER in medium resolution mode we do not detect any molecular emission lines originating from the innermost parts of the disk, such as, e.g., H2O, TiO, or SiO. This result is in contrast to the picture of an inner gaseous disk whose opacity is

caused by molecular emission lines. Recent other studies (Wolf et al., 2012) show as well that the inner gaseous disks are poor in molecules. Thus, the composition of the material inside the inner dust rim remains unclear and probably much more complex than in the previously described picture of an inner gas disk. One possible alternative proposed by, e.g., Benisty et al. (2010a) could be the existence of high refractory grains such as iron, corundum, or graphite, which can survive at higher temperatures than the 1500 K usually used as dust sublimation temperature for silicates and thus closer to the star . The second result of our study is the size of the near-IR emission region in general and in par- ticular the size of the Brγ line emission. The characteristic diameter found for the NIR continuum emission region is determined to be 1.3 – 2 AU, what is in agreement with previously obtained values (Kraus et al., 2008b). As described above, the visibilities show no significant in- or decrease at the wavelengths of Brγ line emission. This means that the Brγ line emission region has a similar size than the continuum emitting region.

From this finding it is possible to draw conclusions on the possible line emission processes. In the magnetospheric accretion scenario matter will fall in along magnetic field lines and will glow brightly very close to the stellar surface. We thus would expect to see the Brγ line emission coming from a very compact region in this case. Our findings point, however, to a much larger line emission region, excluding not only the magnetospheric accretion scenario, but also the X-wind scenario, for which the emission should come from a region of hundredths or tenths of an AU. Therefore, our study supports the disk wind scenario (see also Fig. 6.3). This is in agreement with previous results for Herbig Be stars (Eisner et al., 2004; Malbet et al., 2007; Eisner et al., 2010).

Chapter

7

Summary and Outlook

In this thesis I employed infrared interferometric observations to investigate the formation process of high-mass stars from different points of view. Due to the unprecedented angular resolution interfer- ometric observations in the infrared regime allow us to draw conclusions on the direct circumstellar environment of young stars. Resolving structures on scales between∼ 1 mas and∼ 100 mas they are perfectly suited to shed light on the star formation process from various sides.

One of the possible fields of application for infrared-interferometric observations is to search for companions around young stars. The covered angular separations can close the gap between spec- troscopic and adaptive optics observations. Thus, infrared-interferometric observations are needed to provide a complete (in terms of angular resolution) statistics over the multiplicity rate. Another ap- plication is to follow the orbital motion of a binary system and thus make predictions about its orbital parameters, most important, the stellar masses. I will review the results obtained from the multiplicity studies in Sect. 7.1.

Furthermore, infrared-interferometric observations can provide important information on circum- stellar disks. In order to understand the complex structure of the extended circumstellar disks and envelopes around young stellar objects, spatially resolved observations over an as large as possible wavelength range are required. The hottest material in the innermost (∼ 0.1 to 2 AU) disk regions can be best traced at near-infrared wavelengths, while the warm parts (several hundred Kelvin) of the disk are best studied in the mid-infrared. Such studies are possible with the currently available VLTI instruments AMBER and MIDI, which furthermore provide information about the composition and distribution of the gas and dust due to their spectroscopic capabilities. I will review the results about infrared-interferometric observations in Sect. 7.2.

7.1

Multiplicity

We have used the ESO Very Large Telescope Interferometer to perform long-baseline interferometric observations of a sample of bright stars in the USco OB association and the ONC in the near-infrared. The Orion Nebula Cluster, a part of the Orion OB1 association, and the Upper Scorpius OB asso- ciation are very good targets for such observations, as their stellar content is very well known from spectroscopic as well as adaptive optics searches. Our data are sensitive to companions in the separa- tion range of∼2 to∼100 mas for a brightness ratio≥0.1.

In Upper Scorpius we do not detect any previously unknown companions with our interferometric observations. A clear binary signal in the AMBER visibilities can be detected only for one of the

targets,νSco. We constrain the position of the companion using the AMBER visibilities and closure phase. Combining this new position with additional positions from the literature, we can perform an orbit fit and thus make an estimation for the orbital elements of the system. For the other targets we determine the parameter space in which the presence of companions can be excluded from our data. Furthermore, we used ROSAT X-ray data to search for indications of low-mass companions. For two of the B stars in our sample (πSco and HR 6026), the detection of X-ray emission provides indirect evidence of low-mass companions. The multiplicity for our selected sample is quite high. Including all stars and companions found we find≥2.0 companions per primary star on average for our sample of seven B-type stars (Grellmann et al., 2012A, submitted to A&A).

In the Orion Nebula Cluster we re-observed the already known companions around θ1 Ori C at a distance of ∼ 40 mas and around θ1 Ori A at a separation of ∼ 0.200. The new orbit points for

θ1_{Ori C confirm the predicted orbital period of∼ 11 yrs and the stellar parameters derived from the}

fit. Combining the AMBER data with archival NACO data we can follow the motion of the companion ofθ1Ori A. We conclude that the observed motion is probably due to an highly inclined orbit. For two of our targets, θ1 Ori D andνOri, we find hints for the presence of further companions, which however still need to be confirmed by further observations. Including all possible companions we find∼2.2 companions per primary for our sample of two O- and three B-type stars (Grellmann et al., 2012B, submitted to A&A).

Although for a more complete statistics our samples would have to be extended in particular with regard to the sensitivity, observations searching for companions in the full range of angular distances have been performed for our target stars. It is thus possible to draw conclusions on the multiplicity rate of OB stars. The number of companions per primary found for the selected sample of O- and B-type stars are comparable for the Upper Scorpius association and the Orion Nebula Cluster and are around four times higher than for low-mass stars. This number may even be higher in reality, as all of the techniques used to find companions (e.g., spectroscopic observations, interferometric observations, and adaptive optic surveys) miss, e.g., very faint companions. Our findings are in agreement with the suggestion that the multiplicity rate of young stars is increasing with increasing stellar mass.