If you are interested in the abstract of my PhD thesis from the Star Formation Newsletter No. 143 and COOLNEWS No. 104, have a look to Abstract , or to this other one below from my PhD. thesis directly.
Thüringer Landessternwarte Tautenburg,
Karl-Schwarzschild-Observatorium, Sternwarte 5, D-07778 Tautenburg,
Germany
University: Friedrich-Schiller-Universitaet Jena
Advisor:
Dr. Jochen Eisloeffel (TLS Tautenburg)
Referees:
(1) Prof. Dr. Artie Hatzes (TLS Tautenburg)
(2) Prof. Dr. Karl Menten (MPI fuer Radioastronomy)
(3) Prof. Dr. Rolf Chini (Uni Bochum)
Summary
The nature of young stellar objects (YSOs, Strom~1972), which by definition are sources that have not started stable hydrogen burning, is not yet well understood. Our knowledge about the earliest phases of the protostellar evolution is especially limited because many YSOs are still deeply embedded in their parental dense gas and dust clouds, which makes them difficult to observe and model. However, dramatic improvements in infrared (IR) and millimetre detector technology have enabled astronomers to detect many YSOs that are hidden by dust absorption at optical and near-IR wavelengths. YSOs are classified according to the shape of the emitted spectrum from near- to far-IR spectral regions (Lada \& Wilking~1984; Lada~1987). The sources are divided into three classes by their infrared excess (with respect to the stellar flux): ``Class\,1'' (flux $\lambda F_\lambda$ is larger at longer than at shorter wavelengths), ``Class\,2'' (infrared excess) and ``Class\,3'' (no infrared excess) (Andr\'e \& Montmerle~1994). Andr\'e, Ward-Thompson \& Barsony (1993) identified a new class of YSOs from submillimetre continuum observations, ``Class\,0 objects'', introducing a modification in the IR classification. Class\,0 sources represent the {\em earliest phase} of protostellar evolution. Andr\'e \& Montmerle (1994) suggested the evolutionary sequence Class\,0$\rightarrow$Class\,1$\rightarrow$Class\,2$\rightarrow$Class\,3. Lately, this scheme was even extended to ``Class\,-1'' by the pre-stellar cores (e.g. Boss \& Yorke~1995; Ward-Thompson et al.~2003).
Since Class\,0 sources are highly obscured by extended dusty envelopes, these objects emit mainly at far-IR to millimetre wavelengths. To investigate the physical structure, processes, and properties of Class\,0 sources, six star forming regions were observed in the Perseus and Orion molecular cloud complexes at 450 $\mu$m and 850 $\mu$m with the Submillimetre Common-User Bolometer Array (SCUBA) camera at the James Clerk Maxwell Telescope (JCMT) on Mauna Kea, Hawaii. The target regions were L1448, L1455, NGC\,1333, HH211, L1634, and L1641\,N.
From the new continuum SCUBA maps, 36 submillimetre sources were detected. Some of them are extended, and many contain multiple condensations, as well as extended diffuse features. Among these objects, fifteen sources are analyzed here. The first step of this work has been to characterize these sources. Twelve of these objects are reported here for the first time. Additionally, the first detailed submillimetre observations of NGC\,1333 South and L1641\,N have been obtained and are discussed in this thesis, unveiling new deeply embedded outflow sources. Physical parameters like the dust submillimetre spectral index $\alpha_{450/850}$ (this value is related directly to the dust emissivity exponent $\beta$), gas and dust masses, and sizes of the envelopes are derived. The mean value of $\alpha_{450/850}$ for these objects is 2.8 $\pm$ 0.4, implying that the regions are quite cold ($\sim$10\,K). Such low dust temperatures have for example also been found for isolated globules in the Perseus region (see e.g. Myers \& Benson~1983), suggesting an exponent $\beta \sim\,1$ for the dust grain opacity, $\kappa_{\nu}$. The failure of the Rayleigh-Jeans approximation (failure of the low-frequency portion of the Planck blackbody emission law, for which the dust temperature falls below $h\nu/k$) could increase the value of $\beta$.
The average value of the gas and dust mass of the sample, based on the thermal emission from the dust, is 2.5 $\pm$ 0.6 M$_{\odot}$ (typical aperture 45$''$). Assuming that the observed sources can be fitted by a power-law intensity distribution $I_{\nu}(b)/I_{0}=(b/b_{0})^{-m}$ like Class\,0/1 sources, the average value of $<$$m$$>$ is $1.74\pm 0.02$ at 850 $\mu$m and $1.50 \pm 0.02$ at 450 $\mu$m ($b$ being the impact parameter, the distance between the central source and a position in the envelope where the emission is observed, and $m$ the power-law index mass at the observed frequency). Following the analytic estimate for the intensity (Adams~1991), these values of $<$$m$$>$ correspond to the expected ones for very young objects: theoretical models as well as numerical simulations of the collapse of an isothermal sphere (e.g., Shu~1977; Larson~1969; Penston~1969) also predict power-law indices of the density distribution, $\rho \propto r^{-p}$, with $p \leq$ 2.0. Furthermore, the images revealed the presence of new candidate pre-stellar condensations and/or Class\,0 protostars. In order to derive physical properties of the sources from the observed azimuthally averaged radial intensity profiles of the thermal emission from the dust, like the power-law index of the temperature distribution as a function of the radius $T \propto r^{-q}$, $q$, $p$, and the envelope sizes, the standard envelope model (Adams~1991) was adopted. The values $q$= $0.42\pm0.04$ as well as $p$= $2.1\pm0.1$ and 2.3$\pm$0.1 (at 450~$\mu$m and 850~$\mu$m, respectively) are found. The observed sources are surrounded by extended envelopes, having typical sizes of 1500-6000 AU (at 450 $\mu$m) and 4000-9000 AU (at 850 $\mu$m).
In order to derive more properties of young protostars like the bolometric temperature ($T_{bol}$) and luminosity ($L_{bol}$), the size of the envelope, and the sub-mm slope of the spectral energy distribution (SED), the blackbody fitting technique is used. Accurate SEDs for nine sources are obtained by combining the submillimetre fluxes determined here with values from the literature that cover a broader wavelength range. It turned out that the investigated sample consists of cool objects ($T_{bol}$ ranges from $\sim$27-50 K) which have a $L_{bol}$ of $\sim$4-85 L${_\odot}$. Thus, by means of the sub-mm to bolometric luminosity ratio, L1448\,NW, L1448\,C, RNO\,15\,FIR, NGC\,1333\,IRAS\,1, NGC\,1333\,IRAS\,2, HH211-MM, L\,1634, L\,1641\,N, and L1641\,SMS\,III are Class\,0 sources. A more detailed physical interpretation of the internal physical structure of these objects, however, requires radiative transfer modeling.
The next step is to obtain a more thorough understanding of the internal structure and of the physics of the Class\,0 sources. Therefore an intensive numerical simulation by a radiative transfer code based on the Monte Carlo method for nine representative objects has been used. A spherically symmetric envelope model, and assumptions on the density and dust distributions following the standard envelope model (simple power-law indices $p$ and $\beta$), allowed the observed SED and the radial profiles of the sources to be reproduced. The temperature distribution $T\propto r ^{-q}$ with $q$=0.4 is reproduced for the sample. It is interesting to note that in the inner part of the envelope (10\,AU), the temperature distribution departs significantly from the optically-thin assumption, as well as from a single-power law index $q$=0.4 which was derived observationally. This phenomenon could be interpreted as thermal convection in the inner envelope. In the framework of this model it was also possible to derive other physical parameters of the sources like the radii and the masses of the envelopes, the density distributions, and sublimation radius of the dust. The validity of the results is evaluated from the consistency between the observed and modeled SEDs.
From the modeling of the observed Class\,0 sources, it is concluded that the central engines of these objects have temperatures of typically 3500\,K, and are surrounded by envelopes with masses of about $\sim$1-6 M$_{\odot}$ and sizes between 3000-10000\,AU. The power-law index $p$ spans a range of 1.5-2, which is in agreement with the expectations from all collapse models and numerical simulations. The sublimation radius of 3-5\,AU turned out to be roughly the same as the radius of the photosphere at $\tau$=1 if $p$=2 ($\sim$10\,AU for $p$=1.5). A detailed comparison between the observed and modeled radial profiles reveals that the assumption of spherical symmetry is reasonable. Nevertheless, future models that include outflow and a disk, as well as other geometries would be necessary in order to describe observations at higher spatial resolutions.
The observed properties of the sources can be understood more thoroughly if it is assumed that objects evolve in time. Therefore a last step in this analysis was to determine age, infall rate and envelope masses as a function of time for the nine sources, using a protostellar evolutionary scheme and constructing the $L_{bol}$--$T_{bol}$ diagram for the sample. It is found that the nine sources are quite young ($\sim$10-30 $\times$ 10$^3$ yrs). The density structure apparently evolves from $\rho \propto r^{-2}$ at younger ages to a $\rho \propto r^{-3/2}$ law at later times.
Defense: 9.7.2004