With J. Vinko and G. Kaszas I have carried out a Baade-Wesselink
analysis of 18 Type II. Cepheids. The aim of this project was to make a
homogeneous sample of radii computed with the same method, then to search
for other possibly Type I stars among those classified previously as Type
II, mainly based on their kinematics and galactic position. For this I
wrote a
computer code and applied it to the stars of our sample. As a
result of our research we found five classical like Cepheids (which
have too large radius to be Type II Cepheids) in our sample. Our next goal
is to extend the examination to more Cepheids for which accurate
photometry and spectroscopy are available, and to find an explanation of
how the old-disk population at higher galactic latitudes could produce
these massive classical Cepheids.
The binary Cepheid AU Peg shows a strong Halpha emission line. In
order to determine of the origin of this phenomenom, I am analysing
the spectra of AU Peg taken by J. Vinko. The analysis consists of 3 steps:
#1 The separation of the emission from the absorption using
spectra of a standard star or model atmospheres.
#2 From the equivalent width of the emission I can determine
the mass loss of the star using model calculations.
#3 Since the AU Peg is a binary, we can construct a model for
the mass transfer between the two stars.
I have used IRAF for step #1, and I wrote a computer code for steps
#2 and #3.
In this project we attend to establish a uvby + Hbeta photometric
calibration of the absolute magnitudes of the Cepheid stars in open clusters.
The independent reddenings and distances of these clusters enable the
re-calibration of the period-luminosity relation. Furthermore, accurate
determination of fundamental physical parameters is possible using the
recent synthetic colour grids.
The aims of this project are:
1. Determination of age, distance moduli, metallicities, reddening
of poorly studied open clusters using CCD based multi-color photometry;
2. Discovery of new Cepheids and W UMa stars in these clusters.
Open clusters play a very important role in the study of Galatic structure.
The observation of these objects provide knowledge of physical variables
such as age, metallicity, position which are required if we want to understand
the formation and the early evolutionary phase of our Galaxy.
The age-metallicity diagram of the different regions of the Milky Way
have proven a good tool to test the formation theories.
The color-magnitude diagram allows us to test the theories of stellar evolution and the models derived from them. For example color-magnitude diagram is a good discriminator between overshooting and classical evolutianary models.
Another interesting case would be the discovery of new Cepheids and
contact binaries in these clusters in two points of view:
a) only a few (about 30) Cepheids are known to belong to open clusters
and it would be useful to increase this number;
b) the evolutianry state of contact binaries is quite uncertain and
it would be important to have some observational constraints on their physical
properties.
The first part of our project was carried out in Calar Alto on the summer of 1996. With G. Kaszas I have carried out an observing run with the 1.23m telescope of the German-Spanish Astronomical Centre in Calar Alto, Spain. The aim of our observation was to obtain more data for some fainter, less known open clusters (NGC 7128, NGC 6686, NGC 7086, NGC 7062, NGC 6846), in order to contribute to the observational basis of the field.
In the second part (carried out on the summer of 1997 also in Calar Alto) we extended our dataset that were made in Johnson system with new measurements through Stromgren filters. The program contained compact clusters that are considerably close to the galatic plane. They commonly suffer from high interstellar reddening and some of them have a strong reddening gradient within the cluster. In order to study the problem of reddening applied the intermediate-band photomery. The use of Stromgren uvby-beta-filters made it possible to derive individual reddenings of cluster members. With the help of existing calibrations we can determine the E(B-V) values and de-reddened color indices from the E(b-y) data. The conversion between Johnson and Stromgren data based on simultaneous photoelectric observations of F-G supergiants stars is an ongoing project at our institute (L.Kiss et al., in preparation). The use of individual reddening values would enable us to take into account the variable absorption within the field and this could reduce the scatter on the color-magnitude diagram. It allows us to increase the accuracy of the determination of cluster membership.
During the third part which was carried out in David Dunlap Observatory
on the summer of 1998 we made spectroscopic observation on the brightest
star of one of our candidate cluster
in order to determine accurate spectral type and individual reddening
of these stars.
The original aims of the program mentioned above can then be completed
after the detailed dereddening process. In addition, using precise reddening-free
color indices we could derive fundamental physical parameters of individual
cluster members via model atmospheres.
This would also be useful for the study of the new variables discovered
by us in these clusters. Sometimes it is very difficult to determine the
type of the variables found on short exposure frames. The knowledge of
physical parameters of these stars can help to decide which subgroup
they belong to.
To achieve the above mentioned goals we carried out three observing run in two different observatory (Calar Alto Observatory 1996, 1997, David Dunlap Observatory 1998). In addition I spent a month as a visiting student in the Instituto de Astrofisica de Andalucia, Granada, Spain where I could work together with Dr. Emilio Alfaro on my project on open clusters.
In the future we plan to make further observations in order to obtain
data from
other clusters.
The understandig of star formation is a very important step in the
investigation of our Universe. This field has been a subject of very intense
research that increased our knowledge during the past decades. Now we know
that stars probably form from a molecular cloud when it begins to collapse
under its own gravity. This collapse can be characterized by the free-fall
timescale of the cloud. With the formation of a quasi-static protostar,
the rate of
evolution becomes controlled by the rate at which the star can thermally
adjust to the collapse. The gravitational potential energy liberated by
the collapse is released over time and is the source of the object's luminosity.
This phase
of a star's life is called Pre-Main Sequence (PMS) evolutionary stage.
PMS stars are generally classified into two groups: T-Tauri stars, and
Herbig AB emission line stars (HAeBe) (see Kenyon & Hartmann 1995;
de Winter 1996). The spectral type ranges covered by these groups leave
a gap of significantly less frequent stars, between the intermediate F
types of the hottest T-Tauri stars, and the late A types of the coolest
HAeBes (de Winter 1996). Observationally, PMS stars are defined by
the following criteria. From the
morphological point of view they happen to be occasionally associated
with regions of high extinction, and/or reflection nebulae, both phenomena
frequently found in regions of active or recent star formation. In visual
photometric bands, they appear around 0.5 to 1 mag brighter than their
main-sequence counterparts, indicating a lower surface gravity. In many
cases, mainly for the youngest objects, an excess is found at both near
and far infrared wavelengths. Spectroscopically, they show a variety of
emission features (Balmer lines, CaII H and K lines, forbidden lines of
[OI], [SII], [NII]), or filling the cores of some absorption lines (Halpha,
CaII) (Chavarria 1994; Bohm & Catala 1994,1995; de Winter 1996).
The observational characterization of PMS stars i.e. the establishing
of mass- and age-dependent color-luminosity relations, is a crucial
piece of information to the understanding of the physical processes leading
from molecular clouds to Main Sequence stars. This kind of observational
HR-diagram for PMS stars would make a comparison between observations and
predictions of the evolutionary model s possible, much in the same way
as this comparison is performed for stars of luminosity classes V to I.
In this context, the detection of PMS stars as members
of star clusters offers an advantage over the observation of individual
stars, because isochrone fitting allows a more reliable measurement of
distances and ages for star clusters than for single stars. This may lead
to the obtaining of pre
cise colors and luminosities for the possible PMS members of open clusters
with well defined constraints on age and mass.
We also know that during the fragmentation of the molecular cloud usually more low-mass stars form than high mass ones. This implies that the number of newly formed stars per unit mass interval per unit volume is strongly mass dependent. This functional dependence is known as the initial mass function (IMF). There are many unanswered questions regarding the IMF: Is the mass function the same from cloud to cloud? What is the origin of the mass function? How the mass function might depend on local and gobal condition? Having mass- and age-dependent color-luminosity relations for PMS-stars we can determine the mass function which can be compared to theoretical predictions of cloud collapse models. It would provide us a method for testing the theories of star formation and stellar evolution.
The primary aim of my proposed research is the detection of PMS stars in young galactic open clusters. The three main objectives of this work are
- the improvement of the observational color magnitude relations
for stars of well determined age,
- the enlargement of the PMS sample, in particular for stars
of spectral types around F0-F5
- testing the models of star formation with the help of IMF determined
from the observations of the clusters.
In order to achieve the above mentioned goals I would like to undertake the following observing programs:
1. A photometric study (optical and infrared) of selected bright clusters,
to obtain accurate and homonegeous colors for the stars and to detect possible
IR excess emission which is a good way to identify PMS stars. This consists
of UBVRI CCD photometric observations with the 48" telescope of Whipple
observatory and far-infrared photometry (JHKL) with the dual-beam camera
available at the same place. The most suitable targets for this purpose
are probably close, bright, rich clusters, associated with known star forming
regions such as chi and h Persei, IC 1805,
IC 1848, NGC 6871, that may contain a population of PMS stars in the
brightnes range of 17th - 18th mag that can be detected with these instruments.
2. A deep spectroscopic survey of clusters having PMS candidates with
a multi-object spectrograph such as the planned Hectospec on the converted
MMT. Obtaining large numbers of good quality spectra of the cluster members
we can get reliable spectral types. This would allow us a more accurate
determination of cluster parameters and membership. The planned Hectoechelle
on the same telescope would be useful to search for radial velocity variations
and get masses of binary candidates. This could be particularly interesting,
because it would allow a comparis
on of the masses directly observed with those derived from theoritical
isochrone fitting to the color-magnitude diagrams provided by the photometric
study.
The feasibility of my planned research can be justified in the following
way. PMS stars in open clusters has been searched for almost 50 years.
Since the first pioneering works on NGC~2264 (Walker 1956), NGC~6530 (Walker
1957), and the Orion association (Walker 1969) several clusters have been
investigated for the membership of PMS stars. The most recent studies deal
with NGC6611, (de Winter 1996); IC~380, (Trullols & Jordi 1997); IC~1590,
(Guetter & Turner 1997); IC~4996, (Delgado et al. 1998). My project
is largely based on the results obtained in this last paper, where the
authors found photometric evidences of PMS membership for stars in the
spectral-type gap around F0-F5 mentioned above. These results provide us
a guiding example to elaborate some criteria for
the selection of other clusters. Taking PMS stars around this spectral
types as example, we deal with absolute magnitudes in the range Mv
= 3 - 4. Detection of these stars in a CCD UBV color magnitude diagram
reaching down to V = 18 - 19 therefore restricts the search to clusters
with apparent distance modulus not h
igher than V-Mv = 15. On the other hand, young clusters
associated with regions of active star formation are usually highly affected
by reddening, with typical values of color excess around E(B-V) = 1, which
gives an upper limit for the distance of the candidates of V0-Mv
= 12, or a distance of 2.5 kpc. From the theoretical knowledge achieved
up to now, we can extract some additional restrictions of age (Palla &
Stahler 1993). We have to deal with PMS stars in an age range between about
3*106 yr and 107 yr. PMS stars in younger clusters
are too much embedded in clouds to be detectable at optical wavelengths,
and those in older clusters have already arrived at the main sequence.
Therefore, our sample has to be made out of clusters not older than ten
millioms years,and located at distances closer than 2.5 kpc. Such a sample
is accesible in sections of two spiral arms close to the Sun, in the Cygnus-
and Perseus-arm. Many of the catalogued young galactic clusters, among
them several poorly studied objects, are physically associated to the star
forming regions located in these arms.