10 research outputs found
Impact of photometric variability on age and mass determination of Young Stellar Objects: A case study on Orion Nebula Cluster
In case of pre-main sequence objects, the only way to determine age and mass
is by fitting theoretical isochrones on color-magnitude (alternatively
luminosity-temperature) diagrams. Since young stellar objects exhibit
photometric variability over wide range in magnitude and colors, the age and
mass determined by fitting isochrones is expected to be inaccurate, if not
erroneous. These in turn will badly affect any study carried out on age spread
and process of star formation. Since we have carried out very extensive
photometric observations of the Orion Nebula Cluster (ONC), we decided to use
our multi-band data to explore the influence of variability in determining mass
and age of cluster members. In this study, we get the amplitudes of the
photometric variability in V, R, and I optical bands of a sample of 346 ONC
members and use it to investigate how the variability affects the inferred
masses and ages and if it alone can take account for the age spread among the
ONC members reported by earlier studies. We find that members that show
periodic and smooth photometric rotational modulation have their masses and
ages unaffected by variability. On other hand, we found that members with
periodic but very scattered photometric rotational modulation and members with
irregular variability have their masses and ages significantly affected.
Moreover, using Hertzsprung-Russell (HR) diagrams we find that the observed I
band photometric variability can take account of only a fraction (about 50%) of
the inferred age spread, whereas the V band photometric variability is large
enough to mask any age spread.Comment: Accepted by MNRAS; 17 pages, 4 Tables, 15 Figure
Optical afterglow of the not so dark GRB 021211
We determine Johnson and Cousins photometric CCD magnitudes for
the afterglow of GRB 021211 during the first night after the GRB trigger. The
afterglow was very faint and would have been probably missed if no prompt
observation had been conducted. A fraction of the so-called ``dark'' GRBs may
thus be just ``optically dim'' and require very deep imaging to be detected.
The early-time optical light curve reported by other observers shows prompt
emission with properties similar to that of GRB 990123. Following this, the
afterglow emission from min to days after the burst is
characterized by an overall power-law decay with a slope in the
passband. We derive the value of spectral index in the optical to near-IR
region to be 0.60.2 during 0.13 to 0.8 day after the burst. The flux decay
constant and the spectral slope indicate that optical observations within a day
after the burst lies between cooling frequency and synchrotron maximum
frequency.Comment: 4 pages, 2 figures, A&A Letters, 408, L2
Exploring pre-main sequence variables of ONC: The new variables
Since 2004, we have been engaged in a long-term observing program to monitor
young stellar objects in the Orion Nebula Cluster. We have collected about two
thousands frames in V, R, and I broad-band filters on more than two hundred
nights distributed over five consecutive observing seasons. The high-quality
and time-extended photometric data give us an opportunity to address various
phenomena associated with young stars. The prime motivations of this project
are i) to explore various manifestations of stellar magnetic activity in very
young low-mass stars; ii) to search for new pre-main sequence eclipsing
binaries; and iii) to look for any EXor and FUor like transient activities
associated with YSOs. Since this is the first paper on this program, we give a
detailed description of the science drivers, the observation and the data
reduction strategies as well. In addition to these, we also present a large
number of new periodic variables detected from our first five years of
time-series photometric data. Our study reveals that about 72% of CTTS in our
FoV are periodic, whereas, the percentage of periodic WTTS is just 32%. This
indicates that inhomogeneities patterns on the surface of CTTS of the ONC stars
are much more stable than on WTTS. From our multi-year monitoring campaign we
found that the photometric surveys based on single-season are incapable of
identifying all periodic variables. And any study on evolution of angular
momentum based on single-season surveys must be carried out with caution.Comment: Accepted for publication by MNRAS, 26 pages, 17 figures, 6 table
Results from a spectroscopic survey in the CoRoT fields
Aims.In this paper we report the results of the spectroscopic survey
we carried out to detect chromospheric activity of late-type stars
in the fields of view of the CoRoT space mission.
Methods.We give an accurate MK classification of all targets, which is valuable information on both the main CoRoT project
and additional science programs, by means of cross-correlation with MK standard stars, by using artificial neural
networks. The presence or
absence of excess Hα emission, determined using spectral
subtraction technique, is used to characterize the chromospheric
activity level.
Results.In most cases our MK classification agrees with the spectral
classification reported in the SIMBAD database; however, there are a few
stars that are found to have very different MK classes. Our survey reveals that ~7% of late-type stars
in our sample indeed possess a very active chromosphere. The measured Hα excess and the presence of the 6708 Å Li line allow us to confirm the membership
of 5 targets in the young open cluster NGC 2264