A study of T Tauri stars with multiple spectral types and their surface temperatures

This thesis describes the results of two spectral studies of T Tauri stars. T Tauri stars are young, low mass, stars which have yet to evolve to the point where Hydrogen fusion occurs in their cores. They are normally found in groups associated with gas clouds in locations called star forming regions. Much work has been done to explain the bizarre spectral features of T Tauri stars, and this has led to the development of a standard model for their structures. Classical T Tauri stars are thought to be surrounded by large discs of accreting material, which slowly adds to the mass of the star. This accretion process results in large U.V. and I.R. fluxes for the stars when compared to their main sequence counterparts. Weak-line T Tauri stars do not display any accretion features and are thought to be more evolved than Classical T Tauri stars. Observations show that both types of T Tauri star have active chromospheres and large cool spots, which are both thought to be a result of strong magnetic fields. This thesis is motivated by two studies, one spectroscopic, one photometric, both of which suggest that some T Tauri stars display photospheric features from regions of different temperatures, or multiple spectral types (MST).The first spectral study discussed is of near-I.R. data taken with UKIRT. The spectra are of high resolution data from the region around 1.63 fim, and are of T Tauri, giant and dwarf stars of known spectral type. It is shown that a ratio of selected OH to Fe lines gives a good trend with the optically measured Te//> derived from spectral types, for both dwarfs and giants. Whilst some of the T Tauri stars fit this trend, it is found that 4 stars clearly lie above it. This is taken as an indication of cooler regions in the spectra than had previously been detected, an MST effect.The second spectral study discussed is of optical data taken with the FLAIR system on the UKST. The spectra span the entire optical region observable with this instrument, and are intended to show variation in temperature sensitive features between the blue and red optical regions. Spectra are presented from 2 consecutive nights of observations and previously discovered MST stars are found to show deeper G bands than would be predicted from the TiO bands in the red spectra. It is found that the G band of one object has varied both on a night to night basis and since the previous observations from the mid-1980’s. It is also shown that no variation has occurred in the red spectraAll of the information on MST stars is then compared in order to determine the best physical model for the effect. It is shown that none of the regions in the standard model of Classical T Tauri stars can produce the MTS features. The first model considered that could display MTS features is of binary systems of stars with differing spectral type, but equivalent flux in the optical. It is found that these systems would be too rare to explain all the MTS results. The large cool spots seen on T Tauri stars are also considered as the cause of the MST effect. It is shown that the size of spots required is much greater than the size of observed cool regions. The current theories of solar active regions are discussed, revealing that the dominant photospheric regions are in fact the small, bright faculae. Whilst the spectra of solar faculae are still not fully understood it is found that a faculae+photosphere+cool spot model of T Tauri spectra can produce all the observed MST effects.Finally, the systematic errors in the H-R diagram introduced by the MST effect are considered. By comparison with research on dwarf stars it is found that the MST effects do not significantly alter the stellar Tef f and luminosity. The largest errors are introduced if only a small wavelength region is considered when determining the spectral type of the star. Studies that derive a Te/ / from an apparent spectral type and ignore the MST effect can both over and underestimate Te/ / ; leading to a possible factor of 2 error in the stellar mass. As T Tauri spectral parameters are also used to derive the overall features of star forming regions, this can lead to the miscalculation of the mass function and ages of these regions

The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.Comment: Published in the ESO Messenge