arXiv subjects

J. Tennyson

Publications and source records attributed to J. Tennyson.

63 records · Page 4Linked to original sources

arXiv2006

Non-grey hydrogen burning evolution of sub-Solar mass Population III stars

The primordial elements, H, He and Li are included in a low temperature equation of state and monochromatic opacity function. The equation of state and opacity function are incorporated into the stellar evolution code NG-ELMS, which makes use of a non-grey model atmosphere computed at runtime. NG-ELMS is used to compute stellar evolution models for primordial and lithium free element mixtures, for stars in the sub-Solar mass range 0.8--0.15 Msol. We find that lithium has little or no effect upon the structure and observable properties of of stars in this mass range. Furthermore lithium is completely destroyed by fusion before the main sequence in stars of mass less than ~0.7 Mol. We find that on the red giant branch and Hayashi track, the use of a non-grey model atmosphere to provide the upper boundary conditions for the stellar evolution calculation, results in significantly cooler less luminous stars, across the mass range

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arXiv2006

First Measurements of the Electron Density Enhancements Expected in C-shocks

Magnetic precursors of C-shocks accelerate, compress and heat molecular ions, modifying the kinematics and the physical conditions of the ion fluid with respect to the neutral one. Electron densities are also expected to be significantly enhanced in shock precursors. In this Letter, we present observations of strongly polar ion and neutral molecules such as SiO, H13CO+, HN13C and H13CN, which reveal the electron density enhancements associated with the precursor of the young L1448-mm outflow. While in the ambient gas the excitation of the ions and neutrals is explained by collisional excitation by H2 with a single density of 10E5cmE-3, H13CO+ shows an over excitation in the shock precursor component that requires H2 densities of a factor of >10 larger than those derived from the neutral species. This over excitation in H13CO+ can be explained if we consider an additional excitation by collisions with electrons and an electron density enhancement in the precursor stage by a factor of 500, i.e. a fractional ionization of 5x10E-5. These results show that multiline observations can be used to study the evolution of the ion and electron fluids at the first stages of the C-shock interaction.

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arXiv2006

A high accuracy computed water line list

A computed list of H$_{2}$$^{16}$O infra-red transition frequencies and intensities is presented. The list, BT2, was produced using a discrete variable representation two-step approach for solving the rotation-vibration nuclear motions. It is the most complete water line list in existence, comprising over 500 million transitions (65% more than any other list) and it is also the most accurate (over 90% of all known experimental energy levels are within 0.3 cm$^{-1}$ of the BT2 values). Its accuracy has been confirmed by extensive testing against astronomical and laboratory data. The line list has been used to identify individual water lines in a variety of objects including: comets, sunspots, a brown dwarf and the nova-like object V838 Mon. Comparison of the observed intensities with those generated by BT2 enables physical values to be derived for these objects. The line list can also be used to provide an opacity for models of the atmospheres of M-dwarf stars and assign previously unknown water lines in laboratory spectra.

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arXiv2005

Improved HCN/HNC linelist, model atmospheres and synthetic spectra for WZ Cas

We build an accurate database of 5200 HCN and HNC rotation-vibration energy levels, determined from existing laboratory data. 20~000 energy levels in the Harris et al. (2002) linelist are assigned approximate quantum numbers. These assignments, lab determined energy levels and Harris et al (2002) energy levels are incorporated in to a new energy level list. A new linelist is presented, in which frequencies are computed using the lab determined energy levels where available, and the ab initio energy levels otherwise. The new linelist is then used to compute new model atmospheres and synthetic spectra for the carbon star WZ Cas. This results in better fit to the spectrum of WZ Cas in which the absorption feature at 3.56 micron is reproduced to a higher degree of accuracy than has previously been possible. We improve the reproduction of HCN absorption features by reducing the abundance of Si to [Si/H] = --0.5 dex, however, the strengths of the $Δv=2$ CS band heads are over-predicted.

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arXiv2005

Spectral energy distribution for GJ406

We present results of modelling the bulk of the spectral energy distribution (0.35 - 5 micron) for GJ406 (M6V). Synthetic spectra were calculated using the NextGen, Dusty and Cond model atmospheres and incorporate line lists for H2O, TiO, CrH, FeH, CO, MgH molecules as well as the VALD line list of atomic lines. A comparison of synthetic and observed spectra gives Tef = 2800 +/- 100 K. We determine M$_bol = 12.13 +/- 0.10 for which evolutionary models by Baraffe et al. (2003) suggest an age of around 0.1 -- 0.35 Gyr consistent with its high activity. The age and luminosity of GJ406 correspond to a wide range of plausible masses (0.07 -- 0.1 Msun).

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arXiv2005

Near-Infrared water lines in V838 Monocerotis

V838 Monocerotis had an intriguing, nova-like outburst in January 2002 which has subsequently led to several studies of the object. It is now recognized that the outburst of V838 Mon and its evolution are different from that of a classical nova or other classes of well-known eruptive variables. V838 Mon, along with two other objects that have analogous properties, appears to comprise a new class of eruptive variables. There are limited infrared studies of V838 Mon. Here, we present near-infrared H band (1.5 - 1.75micron) spectra of V838 Mon from late 2002 to the end of 2004. The principal, new result from our work is the detection of several, rotation-vibration lines of water in the H band spectra. The observed water lines have been modeled to first establish that they are indeed due to water. Subsequently the temperature and column densities of the absorbing material, from where the water absorption features originate, are derived. From our analysis, we find that the water features arise from a cool ~750-900 K region around V838 Mon which appears to be gradually cooling with time.

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arXiv2004

The role of HeH+ in cool helium rich white dwarfs

HeH$^+$ is found to be the dominant positive ion over a wide range of temperatures and densities relevant to helium rich white dwarfs. The inclusion of HeH$^+$ in ionization equilibrium computations increases the abundance of free electrons by a significant factor. For temperatures below 8000 K, He$^-$ free-free absorption is increased by up to a factor of 5, by the inclusion of HeH$^+$. Illustrative model atmospheres and spectral energy distributions are computed, which show that HeH$^+$ has a strong effect upon the density and pressure structure of helium rich white dwarfs with teff < 8000 K. The inclusion of HeH$^+$ significantly reddens spectral energy distributions and broad band color indices for models with Teff < 5500 K. This has serious implications for existing model atmospheres, synthetic spectra and cooling curves for helium rich white dwarfs.

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arXiv2003

The effect of the electron donor H3+ on the pre-main and Main Sequence evolution of low mass zero metallicity stars

H3+ has been shown (Lenzuni et al. 1991, ApJS, 76, 759) to be the dominant positive ion, in a zero metallicity gas at low temperature and intermediate to high density. It therefore affects both the number of free electrons and the opacity of the gas. The most recent H3+ partition function (Neale and Tennyson, 1995, ApJ, 454, L169) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H3+ is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000 < T(K) < 9000 and -14 < log10(density [g/cc]) < -2. In the calculation of these opacities we have made use of the latest collision induced absorption data as well as the most recent H3+ partition function and line opacity data. It is shown that these updated and new sources of opacity give rise to a Rosseland mean opacity for a hydrogen-helium gas which is in general greater than that calculated in earlier works. The new opacity data are then used to model the evolution of low mass 0.15-0.8 Mo zero metallicity stars, from pre-Main Sequence collapse to Main Sequence turn-off. To investigate the effect of H3+ on the evolution of low mass zero metallicity stars, we repeat our calculations neglecting H3+ as a source of electrons and line opacity. We find that H3+ can have an effect on the structure and evolution of stars of mass ~0.5 Mo or less. A gray atmosphere is used for the calculation, which is sufficient to demonstrate that H3+ affects the evolution of very low mass stars to a greater degree than previously believed.

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arXiv2003

The identification of HCN and HNC in Carbon Stars: Model Atmospheres, Synthetic Spectra and Fits to Observations in the 2.7-4.0 micron Region

Model carbon star atmospheres and synthetic spectra have been calculated using the recent HCN/HNC vibration rotation linelist of Harris et al. (2002) ApJ, 578, 657. The calculations are repeated using only HCN lines and show that HNC has a significant effect upon the temperature, density and optical depth of a stellar atmosphere. We fit synthetic spectra in the 2.7-4.0 micron region to observed ISO spectra of the carbon stars WZ Cas and TX Psc obtained by Aoki et al. (1998), A&A, 340, 222. These fits allow us to identify absorption by HNC in the spectrum of WZ Cas at 2.8-2.9 microns, and to determine new independent estimates of effective temperature and log(Nc)/log(No). The findings reported here indicate that absorption by both HCN and HNC is needed to fully explain the observed stellar spectra and represent the first identification of HNC in a star. Q branch absorption by the HCN $Δv_2=1$, $Δv_3=1$ and $Δv_1=1$, $Δv_2=-1$ bands at 3.55 and 3.86 microns respectively, are identified in the spectrum of WZ Cas.

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