R. S. Ram

Searching For Chemical Signatures Of Multiple Stellar Populations In The Old, Massive Open Cluster NGC 6791

Galactic open and globular clusters (OCs, GCs) appear to inhabit separate regions of the age-mass plane. However, the transition between them is not easily defined because there is some overlap between high-mass, old OCs and low-mass, young GCs. We are exploring the possibility of a clear-cut separation between OCs and GCs using an abundance feature that has been found so far only in GCs: (anti)correlations between light elements. Among the coupled abundance trends, the Na-O anticorrelation is the most widely studied. These anticorrelations are the signature of self-enrichment, i.e., of a formation mechanism that implies multiple generations of stars. Here we concentrate on the old, massive, metal-rich OC NGC 6791. We analyzed archival Keck/HIRES spectra of 15 NGC 6791 main sequence turn-off and evolved stars, concentrating on the derivation of C, N, O, and Na abundances. We also used WIYN/Hydra spectra of 21 evolved stars (one is in common). Given the spectral complexity of the very metal-rich NGC 6791 stars, we employed spectrum synthesis to measure most of the abundances. We confirmed the cluster super-solar metallicity and abundances of Ca and Ni that have been derived in past studies. More importantly, we did not detect any significant star-to-star abundance dispersion in C, N, O and Na. Based on the absence of a clear Na-O anticorrelation, NGC 6791 can still be considered a true OC, hosting a single generation of stars, and not a low-mass GC.

EINSTEIN A COEFFICIENTS AND OSCILLATOR STRENGTHS FOR THE A 2 Π-X 2 Σ + (RED) AND B 2 Σ + -X 2 Σ + (VIOLET) SYSTEMS AND ROVIBRATIONAL TRANSITIONS IN THE X 2 Σ + STATE OF CN

Line strengths have been calculated in the form of Einstein A coefficients and f-values for a large number of bands of the A 2Π-X 2Σ+ and B 2Σ+-X 2Σ+ systems and rovibrational transitions within the X 2Σ+ state of CN using Westerns PGOPHER program. The J dependence of the transition dipole moment matrix elements (the Herman-Wallis effect) has been taken into account. Rydberg-Klein-Rees potential energy functions for the A 2Π, B 2Σ+ , and X 2Σ+ states were computed using spectroscopic constants from the A 2Π-X 2Σ+ and B 2Σ+-X 2Σ+ transitions. New electronic transition dipole moment functions for these systems and a dipole moment function for the X 2Σ+ state were generated from high level ab initio calculations and have been used in Le Roys LEVEL program to produce transition dipole moment matrix elements (including their J dependence) for a large number of vibrational bands. The program PGOPHER was used to calculate Einstein A coefficients, and a line list was generated containing the observed and calculated wavenumbers, Einstein A coefficients and f-values for 290 bands of the A 2Π-X 2Σ+ transition with v = 0-22, v = 0-15, 250 bands of the B 2Σ+-X 2Σ+ transition with v = 0-15, v = 0-15 and 120 bands of the rovibrational transitions within the X 2Σ+ state with v = 0-15. The Einstein A coefficients have been used to compute radiative lifetimes of several vibrational levels of the A 2Π and B 2Σ+ states and the values compared with those available from previous experimental and theoretical studies.

IMPROVED LINE DATA FOR THE SWAN SYSTEM 12C13C ISOTOPOLOGUE

We present new, accurate predictions for rotational line positions, excitation energies, and transition probabilities of the 12C13C isotopologue Swan d3Π-a3Π system 0-0, 0–1, 0–2, 1–0, 1–1, 1–2, 2–0, 2–1, and 2–2 vibrational bands. The line positions and energy levels were predicted through new analyses of published laboratory data for the 12C13C lines. Transition probabilities were derived from recent computations of transition dipole moments and related quantities. The 12C13C line data were combined with similar data for 12C2, reported in a companion paper, and applied to produce synthetic spectra of carbon-rich metal-poor stars that have strong C2 Swan bands. The matches between synthesized and observed spectra were used to estimate band head positions for a few of the 12C13C vibrational bands and to verify that the new computed line data match observed spectra. The much weaker C2 lines of the bright red giant Arcturus were also synthesized in the band head regions.

The Magnesium Isotopologues of MgH in the A 2Π-X 2Σ+ System

Using laboratory hollow cathode spectra we have identified lines of the less common magnesium isotopologues of MgH, 25MgH and 26MgH, in the A 2Π-X 2Σ+ system. Based on the previous analysis of 24MgH, molecular lines have been measured and molecular constants derived for 25MgH and 26MgH. Term values and linelists, in both wavenumber and wavelength units, are presented. The A 2Π-X 2Σ+ system of MgH is important for measuring the magnesium isotope ratios in stars. Examples of analysis using the new linelists to derive the Mg isotope ratio in a metal poor dwarf and giant are shown.

LINE LISTS FOR THE A 2Π-X 2Σ+ (RED) AND B 2Σ+-X 2Σ+ (VIOLET) SYSTEMS OF CN, 13C14N, AND 12C15N, AND APPLICATION TO ASTRONOMICAL SPECTRA

New red and violet system line lists for the CN isotopologues 13C14N and 12C15N have been generated. These new transition data are combined with those previously derived for 12C14N, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright very low-metallicity star HD 122563, and carbon-enhanced metal-poor stars HD 196944 and HD 201626. When lines of both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work generally are also in accord with published values.New red and violet system line lists for the CN isotopologues {sup 13}C{sup 14}N and {sup 12}C{sup 15}N have been generated. These new transition data are combined with those previously derived for {sup 12}C{sup 14}N, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright, very low-metallicity star HD 122563, and the carbon-enhanced metal-poor stars HD 196944 and HD 201626. When both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work are also generally in accord with published values.

FOURIER TRANSFORM EMISSION SPECTROSCOPY OF THEA2Π-X2Σ+(RED) SYSTEM OF13C14N

Emission spectra of the A 2?-X 2?+ transition (red system) of 13C14N have been measured in the 4000-15,000 cm?1 region using the Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the National Solar Observatory. The 13C14N free radical was produced in microwave discharge of a mixture of 13CH4 and 14N2. Rotational analysis of 22 vibrational bands involving vibrational levels up to v = 8 and v = 5 of the excited and ground states has been obtained and much improved spectroscopic constants have been determined. An experimental line list and calculated term values are provided. The results of the present analysis are useful for the identification of 13C14N lines in late-type stars in the red and near-infrared spectral regions.

FOURIER TRANSFORM EMISSION SPECTROSCOPY OF THE A {sup 2}{Pi}-X {sup 2}{Sigma}{sup +} (RED) SYSTEM OF {sup 13}C{sup 14}N

Emission spectra of the A 2?-X 2?+ transition (red system) of 13C14N have been measured in the 4000-15,000 cm?1 region using the Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the National Solar Observatory. The 13C14N free radical was produced in microwave discharge of a mixture of 13CH4 and 14N2. Rotational analysis of 22 vibrational bands involving vibrational levels up to v = 8 and v = 5 of the excited and ground states has been obtained and much improved spectroscopic constants have been determined. An experimental line list and calculated term values are provided. The results of the present analysis are useful for the identification of 13C14N lines in late-type stars in the red and near-infrared spectral regions.