| dc.description.abstract | Stars are born predominantly within dense cores of giant molecular clouds having number densities > 10? cm?³. This premise is strongly supported by a wealth of observations showing that the hallmarks of newly formed stars-such as enhanced far?infrared emission and compact regions of ionized gas—are intimately associated with warm and dense regions of molecular gas.
Following the paradigm of low?mass (< 2 M?) star formation (Shu, Adams and Lizano 1987), we understand that the central region of a dense core begins to condense quasi?statically through ambipolar diffusion. As magnetic pressure support decreases, the central region reaches an unstable quasi?equilibrium state in which thermal pressure alone supports the core against self?gravity. This stage marks the initial condition for dynamical collapse.
The ensuing major evolutionary stages in the model are:
(a) an accretion stage characterized by a central protostar and a circumstellar disk, surrounded by an infalling envelope of dust and gas;
(b) a phase in which the protostar injects linear and angular momentum and mechanical energy into its surroundings through jets and molecular outflows; and
(c) a more advanced stage in which the protostar settles onto the Zero?Age Main Sequence (ZAMS), increasing its luminosity.
Although this paradigm is successful for low?mass star formation (Lada 1991), its applicability to massive stars is uncertain. Massive stars evolve more rapidly: their Kelvin–Helmholtz timescale (< 10? yrs for an O?type star) is extremely short. They reach the main sequence while still accreting mass from their envelopes. The formation of massive disks, and thus the appearance of jets and molecular outflows, is therefore unclear. In stage (c), massive stars produce intense UV radiation and strong winds that drastically alter the physical conditions, structure, and chemistry of their environment.
Understanding the physical processes dominating the early stages of massive?star formation requires detailed knowledge of the surrounding environment before and after protostar formation. Once formed, a protostar heats the surrounding gas and dust. Dust grains-typically sub?micron silicate, graphite, SiC, etc.-absorb most optical and UV photons from deep within the core, and re?emit as a greybody in the mid? and far?infrared. Because dust opacity decreases at longer wavelengths, far?infrared observations provide excellent probes of the regions closest to the protostar.
Ionizing photons from the star form compact H?II regions, which emit radio free?free radiation. Fine?structure lines from heavier elements (mostly in the infrared) cool the ionized gas. Thus, far?infrared dust emission provides information on dust temperature, composition, and distribution, while radio and spectroscopic infrared observations give corresponding information for the gas.
When massive OB associations illuminate dense clouds, photon?dominated regions (PDRs) form. PDRs contain primarily neutral hydrogen, whereas species such as carbon and oxygen appear in multiple ionization states, becoming neutral or molecular (H?, CO, O?, etc.) deeper into the cloud. Gas heating arises from the photoelectric effect on grains and PAHs, and far?UV pumping of H?. Cooling occurs via:
(i) fine?structure lines, e.g., [C?II] 158 ?m, [O?I] 63 ?m and 146 ?m, [Si?II] 35 ?m, [C?I] 609 and 370 ?m;
(ii) H? ro?vibrational lines;
(iii) CO rotational lines.
Among these, [C?II] 158 ?m and [O?I] 63 ?m are dominant coolants. Their combined intensities correlate well with far?infrared continuum emission, allowing estimates of gas–dust coupling.
Balloon?Borne Far?Infrared Observations
The work in this thesis is based on observations of Galactic star?forming regions aimed at:
Photometric mapping in two far?infrared bands
Mapping of the 157.7409 ?m [C?II] line
These observations were performed using the TIFR balloon?borne FIR telescope at ~31 km altitude. The telescope (flown from TIFR Balloon Facility, Hyderabad) is an f/8 Cassegrain with a 100?cm primary mirror and a 27?cm secondary mirror, chopped at 10 Hz to remove sky background. Raster scans of target fields were performed, with real?time pointing controlled from the ground. Aspect reconstruction was enhanced via optical photometer signals (Naik et al. 2000).
Photometric Observations and Analysis
Observations were carried out in bands centered near 140 ?m and 190 ?m using a two?band photometer containing two arrays of composite silicon bolometers (cooled to 0.3 K). Each array has 6 detectors in a 3×2 geometry with a 1.6? field of view per element. A cooled dichroic beam?splitter enabled simultaneous two?band imaging. Spectral responses were measured using a laboratory Michelson interferometer.
Raw chopped signals were calibrated using planetary observations (for flux and PSF). Data reduction involves:
Aspect reconstruction
Detector gain correction
Gridding into 0.3? × 0.3? pixels
Deconvolution using the PSF via a Maximum Entropy Method (MEM) (Ghosh et al. 1988)
Dust Temperature and Optical Depth
Since both bands map the same sky area with identical beams, pixel?by?pixel intensity ratios yield dust temperature (T?). Only pixels with signal >5? are used. Maps are smoothed with a 3×3 running average to ensure conservative structure recovery.
For optically thin emission:
F? = ? B?(Td) ??
so the ratio of fluxes at two wavelengths depends only on Td (for an assumed emissivity exponent ?). Once Td is known, optical depth ?? follows.
Radiation Transfer Modeling
A typical star?forming region is modeled as a spherical cloud of gas and dust with an embedded protostar. The radiative transfer model (Mookerjea and Ghosh 1999) includes:
equilibrium dust heating
absorption
multiple, anisotropic scattering
multi?grain populations
Model inputs include cloud size, dust density profile, and stellar characteristics. Outputs include:
Spectral Energy Distribution (SED)
Radial intensity profiles
Radio continuum emission from ionized gas
Best?fit models are derived by matching FIR?derived dust temperature maps and observed SEDs.
Two types of photometric studies are presented:
Detailed study of an isolated source, combining temperature maps and radiative?transfer modeling.
Large?scale mapping of a star?forming complex to identify cold cores, estimate emissivity exponents, and derive dust temperature and optical?depth distributions. | |
