Working Group Members:

• H.C. Bhatt, IIA, Bangalore

• N.K. Rao, IIA, Bangalore

• Gajendra Pandey, IIA, Bangalore

• Ranjan Gupta, IUCAA, Pune

• Gopal Krishna, NCRA, Pune

• Paul Groot, Radboud University, The Netherlands

• Noah Brosch, Tel Aviv University, Israel

A number of research areas in stellar astronomy have been identified that will be studied with priority using the TAUVEX observations. Such broad research area as Galactic Astronomy is subdivided into several research topics. The following are:  

---Classification of TAUVEX Sources

TAUVEX sources: classification

Principal Investigator: Ranjan Gupta (IUCAA)

Most stellar targets are point sources for the TAUVEX resolutions. There will also be objects of interest that are spatially extended. In the utilisation of TAUVEX data, common to all studies, one of the first steps would be source classification; morphological and spectral.

This will be the foremost requirement on the processed images of the TAUVEX basic data. The image fields will contain both point sources (stars, QSO's etc) as well as extended sources like nebulae, galaxies, clusters etc. The TAUVEX instrument PSF can be used to make the morphological classification of sources. Flux measurements in different bands and colour-colour diagrams can be used to separate objects with different spectral energy distributions. The Artificial Neural Networks (ANN) based techniques may be used for the accomplishment of these goals.

The group at IUCAA (Ranjan Gupta and his collaborator H.P. Singh from Delhi University) have developed the ANN tool over more than a decade. They have successfully applied this tool to classification of stellar spectra in the UV, Optical and NIR regions. Initial efforts were to use the available digital spectral libraries (in 1994) and obtain two sub-spectral type level of classification accuracies and were the first ones to provide 2D classification of optical spectra (luminosity class and spectral type). The ANN tool was soon improved to render one sub-spectral type classification accuracy in the IUE-UV spectra. Another level of classification (3rd dimension) was possible in a hierarchical fashion i.e. interstellar extinction estimate or the color excess was found from the IUE-UV database.

Later a PCA based pre-processor was developed for use of ANNs on very large databases and to reduce the computation times. The ANN was recently applied to a large near-IR (IRAS spectra) database of 2000 stars and also to the large INDO-US CFLIB spectral library. There have been attempts to fill the flux gaps in various spectral libraries that are available.

Currently we are in the process of using ANNs for simulated TAUVEX filter band fluxes data and its classification with IUE spectra reduced to respective TAUVEX band fluxes. This will help in the classification of point source spectral types once TAUVEX data becomes online.

Stellar astronomy science areas:

Principal Investigator: Harish Bhatt (IIA)

Young Stellar (and substellar) Objects (YSOs) in star forming regions
Young stellar objects (eg. premain-sequence T Tauri stars, Herbig Ae/Be stars) found in and around star forming regions emit excess ultraviolet radiation that arises due to (a) accretion of matter and/or (b) unusually high magnetospheric activity in the convective atmosphere of the star. Young substellar objects like the brown dwarfs and giant exoplanets in star forming regions are also expected to behave similarly. In addition to a general high level of UV emission, there are intense flare events which are expected to be more prominent in the UV. TAUVEX will be used to observe a number of galactic star forming regions that contain YSOs at different stages of pre-main-sequence evolution to study the processes involved in the formation of stars.

Young stars are also known to generally go through a phase during which they eject matter at relatively high velocity ( 100 - 300 km/s) in the form of narrow jet-like bipolar outflows. Some of these extend to large distances from the high density cloud cores in which they form and interact with the lower density intra-cloud or interstellar gas. The Giant HH flows are examples of such objects. While the HH objects are sources of intense line emission, one expects a rising ultraviolet continuum from the interaction surface between the high velocity outflow gas and the effectively stationary, ambient gas where the temperature could reach values as high as 10⁵ - 10⁶ K. This process could in fact be very significant source of galactic ultraviolet emission. TAUVEX will be used to image the Giant HH outflows (typically, 10 - 50 arc min in size) that are in relatively low extinction regions in the ultraviolet. This study will lead to a better understanding of the way young stars disrupt their parent clous and provide a new dignostic tool to study the environments of these objects.

Flare stars
Low mass dMe stars can give rise to strong flares in the ultraviolet with brightenings as large as 5-10 magnitude. Many of of the brightest time-variable objects in the Galaxy Evolution Explorer (GALEX) ultraviolet variability (GUVV) catalog, which contains information on 84 time-variable and transient sources are nearby M-dwarf flare stars. Their flare events can be of durations as short as a few 100 seconds. For such events TAUVEX will be able to obtain flare light curves even in a single scan.

Novae
Novae are cataclysmic binaries (white dwarf + red dwarf) that give rise to outbursts due to runaway thermonuclear reactions in the accreted material on the white dwarf's surface. Recent CHANDRA results have shown that most novae are bright in X-rays a few months after the optical outburst. As the nova brightness in the optical declines and ejectted matter expands and thins out exposing the central hot regions, one may expect to detect UV emission. UV light curves for galactic novae, with simultaneous ground based optical monitoring, will be obtained with TAUVEX.

Survey of hot evolved stars in galactic open clusters
The evolution of intermediate mass stars can be rapid enough for their remnants (central stars of planetary nebulae (CSPN), white dwarfs) to be found still associated with the star clusters in which they were born. The CSPN and young hot white dwarfs will be strong UV emitters. Deep imaging survey of open clusters with TAUVEX will reveal these objects. Their study will throw important light on the evolution of intermediate mass stars and planetary nebulae.

Wolf-Rayet (WR) stars
WR stars represent some of the hottest and most luminous stars in the Galaxy. They would be bright in the UV. Due to their bright UV magnitudes they can be detected to large distances even in the presence of interstellar extinction. Their UV observations will be useful in the study of galactic spiral structure and extinction.

Galactic black hole candidates
Galactic black hole candidates are X-ray binaries that have strong winds and emit at wavelengths from radio to X-rays due to accretion processes and jets. It is proposed to make a multiwavelength study, including UV with TAUVEX, of a number of these objects (eg. SS 433, GRS 1915+105).

Galactic Plane Survey

Principal Investigator: Harish Bhatt (IIA)

Co-PI: Paul Groot (Radboud University)

The aim is to obtain a homogeneous and complete sample of white dwarf (in binaries) systems to be able to better determine their space densities and population characteristics. These we want to compare with population synthesis programs to fill in the gaps in our understanding of e.g. the common-envelope evolution. Another interest of these systems is that the short period AM CVn stars are the best candidates for the direct detection of gravitational waves with LISA.

To obtain these homogeneous samples the optical wide field surveys, e.g. the Sloan survey, are used. The major drawback of most wide field surveys is the fact that they are located at high galactic latitude, which makes it very difficult to see the bulk of the Galactic population, since these are located at low Galactic latitude. To remedy this a number of programs on the Isaac Newton telescope on La Palma were started and soon to start on the VST telescope at ESO Paranal to survey the full Galactic plane (at |b| less 5) in the optical bands, U, G, R, I, and H_alpha. In the North we will also survey the area in the He I 5875 band. This enormous dataset (>> the SDSS) will of course be very interesting to find these white dwarf binaries, but a real extra benefit would be a UV survey of the same area.

Most of these white dwarf systems are relatively hot (T>10000 K), but faint (M(V) > 8). This is therefore a very local, but faint population that will have the peak of their spectral energy distribution in the UV. Although we may be able to identify them from just the optical data, this will always leave large questions on e.g. their temperatures, without any UV data.

The possibility to get UV data in TAUVEX SF1, SF2 and SF3 filters to complement the optical data really would make an ideal complement to Galactic Plane work. With the addition of future nIR surveys with e.g. VISTA, this would make an absolute wonderful dataset on the Galactic Plane, which so far has been remarkably ignored.

Of course there are a number of issues to consider, most prominently of course the spatial resolution of 6"-8" of TAUVEX and the strong absorption by dust in the plane. The first should not be a problem, since a deep integration with TAUVEX field of view of 0.9 degrees, should detect more than one UV-excess object. This, combined with previous optical data, will allow us to set the relative astrometry to an accuracy good enough to uniquely identify which object is which. The second problem is dust. But since we are interested in a very local population (d much less 1 kpc), the effect of dust is relatively mild, which would have to be checked in flight.

Observations of variable stars and stellar systems

Principal Investigator: Gopal Krishna (NCRA)

This long-term project will focus on UV bright stars, particularly stellar systems with an accretion disk, which are expected to undergo flaring in UV/optical bands due to a variety of physical processes whose nature and role is not yet well understood. The major population of this kind are the so-called Cataclysmic Variables (CVs). These are accreting binaries, containing a white dwarf and a late main-sequence star in a close pair, with hour-like orbital periods. They are particularly efficient and variable emitters in the UV band. Presently, ~1500 CVs are known at high galactic latitudes and more are being discovered by combining the ongoing GALEX and SDSS surveys.

The basic aim of this project is to use TAUVEX (in its normal survey mode itself) to systematically pick out of the known high-latitude CVs, those that happen to be in either a flaring, or quiescent state as compared to their past detections. Attempt will also be made to provide simultaneous multi-color optical coverage using the 1-2 m telescopes available in the country. The most exciting CVs thus found will be followed up with optical spectroscopy, as well as in other wavebands, preparatory to their UV/X-ray observations with ASTROSAT.

Solar System Studies

Principal Investigator: Noah Brosch (TA University)

Possible objects could be:
---Bright Comets to follow up;
---Asteroids that would pass through TAUVEX LOS;
---Mapping the Moon in different filters and at different phases. This would be the project for the second year and with minerological implications, that was never properly done before. This project may also complement Chanandraayan;
---Planets.

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