Dusty massive stars in the Local Group

Doctoral Dissertation uoadl:1309617 649 Read counter

Unit:
Τομέας Αστροφυσικής, Αστρονομίας και Μηχανικής
Library of the School of Science
Deposit date:
2016-01-25
Year:
2016
Author:
Britavskiy Mikola
Dissertation committee:
Δέσποινα Χατζηδημητρίου Αναπλ. Καθηγήτρια (Επιβλέπουσα), Άλκηστη Μπονάνου, Παναγιώτης Μπούμης
Original Title:
Dusty massive stars in the Local Group
Languages:
English
Translated title:
Aστέρες Mεγάλης Mάζας με Σκόνη σε Γαλαξίες της Τοπικής Ομάδας Γαλαξιών
Summary:
Τhe role of episodic mass loss in massive star evolution is one of the most
important open questions of current stellar evolution theory. Episodic mass
loss produces dust and therefore causes evolved massive stars to be very
luminous in the mid-infrared and dim at optical wavelengths. Star forming dwarf
irregular (dIrr) galaxies serve as ideal laboratories for investigating the
evolution and mass loss phenomenon of red supergiants (RSGs), supergiants B[e]
(sgB[e]), and luminous blue variables (LBVs) within the context of different
metallicities of host galaxies. Also, RSGs may be used for abundance
determinations in dIrrs. The extremely low number of spectroscopically
confirmed RSGs and other types of dusty massive stars mentioned before, in
external galaxies makes the identification of new stars of these types
statistically significant. Increasing the statistics of spectroscopically
confirmed evolved massive stars in the Local Group enables the investigation of
the mass loss phenomena that occur in these stars in the late stages of their
evolution.
We aim to complete the census of luminous mid-IR sources in star-forming dwarf
irregular galaxies of the Local Group. To achieve this we employed mid-IR
photometric selection criteria to identify evolved massive stars, such as RSGs
and luminous blue variables (LBVs), by using the fact that these types of stars
have infrared excess due to dust.
The method is based on 3.6 $\mu$m and 4.5 $\mu$m photometry from archival {\it
Spitzer} Space Telescope images of nearby galaxies and $J-$band photometry from
2MASS. We applied our criteria to 7 dIrr galaxies: Pegasus, Phoenix, Sextans A,
Sextans B, WLM, IC 10 and IC 1613 selecting 124 point sources, which we
observed with the VLT/FORS2, GTC/OSIRIS and duPont/WFCCD spectrographs in
multi-object and long-slit spectroscopy modes.
We identified 28 RSGs, of which 21 are new discoveries, also 2 new emission
line stars, and 8 carbon stars. Among the other observed objects we identified
foreground giants, and background objects, such as a quasar and an early-type
galaxy that contaminate our survey. We use the results of our spectroscopic
survey to revise the mid-IR and optical selection criteria for identifying RSGs
from photometric measurements. The optical selection criteria are more
efficient in separating extragalactic RSGs from foreground giants than mid-IR
selection criteria, however the mid-IR selection criteria are useful for
identifying dusty stars in the Local Group. For some of the newly identified
RSGs we measured the fundamental physical parameters by fitting their
observational spectral energy distributions with MARCS stellar atmosphere
models. This work serves as a basis for further investigation of the newly
discovered dusty massive stars and their host galaxies.
This PhD thesis is organized as follows: Chapter 1 provides the scientific
context of the present study. In Chapter 2 the procedure for selecting sources
is presented. Descriptions of the observations and data reduction are presented
in Chapter 3. Chapter 4 is devoted to the spectral analysis and the spectral
type classification of the obtained spectra. Chapter 5 is devoted to presenting
and discussing the results. Conclusions and future perspectives of the PhD
thesis work are presented in Chapter 6. In Chapter 7 the acknowledgments and
list of supporting materials (published papers, information about allocated
telescope time, etc.) are presented.
Keywords:
Galaxies, Local Group, Massive stars, Dust, Optical spectroscopy
Index:
No
Number of index pages:
0
Contains images:
Yes
Number of references:
135
Number of pages:
139
document.PDF (8 MB) Open in new window