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Identification and characterization of young stars in star forming regions a thesis submitted to Pt. Ravishankar Shukla University, Raipur for the degree of doctor of philosophy in physics

Saha, Piyali

Identification and characterization of young stars in star forming regions a thesis submitted to Pt. Ravishankar Shukla University, Raipur for the degree of doctor of philosophy in physics [Thesis] Piyali Saha - Bangalore Indian Institute of Astrophysics 2021 - l, 296p.

Star formation is a chain of processes in which molecular clouds get con-
verted to a compact stellar systems, which are the potential platforms

of life. Although, numerous observations and simulations have been per-
formed over few decades to understand these processes and evolutionary

sequences, many questions remain unanswered. Several factors are found

to regulate the star formation processes, e.g. magnetic field and turbu-
lence of the molecular clouds, supernova explosions, high stellar winds

and so on. The formation and interaction of low and high mass stars can

be constructive as well as destructive in small and large scale environ-
ments. In the very early stage of its formation, the young stellar object

(YSO) shows emission in millimeter and sub-millimeter. Accretion of
circumstellar material and outflow are the signatures of interaction of
YSOs with the surroundings. Being associated with the circumstellar

disks, YSOs are often found to have infrared (IR) excess. Due to pres-
ence of accretion, YSOs show emission in Hα, Hβ, Bracketγ, Paschenβ

etc. in their spectra. Due to accretion shocks, YSOs can be associated
with X-ray and ultraviolet emission also. Being formed together, YSOs
share similar kinematic properties as they form together in molecular
clouds. Based on these ways, YSOs can be identified to study the star
formation history in the molecular clouds. In this thesis, we identified
and characterized young stars in star forming regions. We made a detailed study of an elongated cloud named LDN 1172/1174,

showing “head-tail” structure which is located in the Cepheus constel-
lation. LDN 1174 (NGC 7023) is illuminated by a Herbig Be star HD

200775. At first we made an attempt to estimate the distance to this
cloud as it is not well estimated in previous literature. Based on the
latest Gaia DR2 distances of the YSOs associated with LDN 1172/1174 we computed its distance as 335 ± 11 pc. We further confirm the esti-
mated distance to this cloud complex based on the polarization measure-
ments of the stars projected background to the cloud. In the polarizaion

vs. distance (obtained from Gaia DR2) plot of these observed stars we
found a hike in polarization at ∼ 335 pc. Now, based on the median
absolute deviation of the distance and proper motion values of the YSOs
we found 20 additional sources having similar kinematics as the YSOs.
These sources could possibly be YSOs missed in the earlier studies. They
are having little or no near-IR excess and some of them are associated
with X-ray emission. Optical spectroscopic observations of four of them
show signature of Hα in emission. We classify them based on their IR
spectral index. We found that these comoving sources are of 1 − 2 Myr
in age. Finally, we made a census of YSOs towards HD 20075, which is
a Herbig Be star responsible to ionize LDN 1174. We also made near-IR
spectoscopic study towards 10 YSOs to obtain a detailed knowledge of

star formation in LDN 1172/1174. We estimated their physical proper-
ties and found that star formation in this cloud complex started prior to

the birth of the central ionzing source HD 200775.
Magnetic field plays a significant role in evolution of molecular clouds
and regulation of star formation in them. Therefore, we estimated the
orientation and strength of the projected magnetic field BPOS towards
LDN 1172/1174 complex by optical R-band polarimetric observations.
We found that BPOS follows the cloud structure. We compared our
findings with the Planck results also and found that the inferred (BPOS)
is well corelated in optical R-band and Planck observations.
We extend the work done in LDN 1172/1174 on a wider scale in the
Cepheus Flare. There are two more Herbig AeBe stars (BD+68◦1118 and
PV Cep) within 1.5

radius of HD 200775 which share similar kinematics.
We found an age sequence in the stellar associations from BD+68◦1118
(∼ 10 Myr), HD 200775 (∼ 1 Myr) and PV Cep (∼ 0.1 Myr). Based
on the distance and proper motion distribution of the previously known
YSOs from Gaia DR2, we found a loosely bound cluster surrounding
BD+68◦1118 which contains relatively older cluster members whereas
PV Cep is not associated with any cluster. The comoving sources are mainly low-mass M-type sources and have lack of IR excess which is
consistent with their ages.
The young intermediate-mass sources located in the Perseus OB2 region
were observed spectroscopically with high resolution for investigating

their spectral types with more accuracy. These sources don’t have near-
IR excess and emission in Balmer lines. Most of their ages are lying

within 2 − 4 Myr and yet they are having lack of circumstellar disks.
The typical disk dispersal timescale for intermediate-mass stars (3 − 7
Myr) is comparable with their ages due to which it is suspected that
these sources have aleady shreaded their surrounding disks. We present
temperature, extinction, mass, age, and other properties of the observed
sources. We also explain their kinematic properties using the latest Gaia
DR2 and have found these sources are widely scattered across the region.
Some of these sources are kinematically asscoiated with the low-mass
The intermediate-mass Herbig Ae and Be (HAeBe) stars bridge the gap
between low-mass stars, whose formation is fairly well understood and
high mass stars, whose formation still poses challenges. Lower mass stars
are formed through magnetospheric accretion, whereas high mass stars
are thought to form through disk accretion. Linear spectropolarimetry
is an excellent tool to probe the circumstellar disks around stars on
scales of order stellar radii, i.e. the accretion region of young stars.
Now, in case of HBe stars one can observe a depolarization indicaing
disk accretion while in HAe stars line polarization can be seen signifying
magnetospheric accretion. We acquired spectropolarimetric data of six
HAeBe stars and investigated about their accretion mechansims.
Based on the optical polarimetric study of the YSOs associated with
BRC 18, we found that polarization measurements of these sources are
quite random with respect to the cloud’s projected magnetic field. This

finding indicates that the circumstellar disks of the YSOs are also ori-
ented arbitrarily. Based on the kinematic properties of the associated

YSOs using the latest Gaia DR2, we showed that BRC 18 and most of
the other BRCs are moving away from the direction of ionization, which

is a signature of the “Rocket Effect”. We have found a strong corre-
lation between the angle of ionizing source with the IRAS source and the proper motion of the YSOs with respect to the celestial north-south

axis. In addition, we searched for sources having projected motion sim-
ilar to the YSOs and could possibly be young sources not detected in

earlier surveys. A fraction of them show near-IR excess and similar age
distribution as the known YSOs and candidates.

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