LTE Analysis Research Articles

High resolution spectroscopy of stars with transiting planets

Context: During the past years photometric surveys, later complemented by follow-up radial-velocity measurements, have revealed the presence of several new extra-solar transiting planets, in very short period orbits. Many of the host stars are extremely faint (V$\sim$16), making high-precision spectroscopic measurements challenging. Aims: We have used the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of 5 stars hosting transiting planets, namely for OGLE-TR-10, 56, 111, 113 and TrES-1. The immediate objective is to derive accurate stellar parameters and chemical abundances. Methods: The stellar parameters were derived from an LTE analysis of a set of FeI and FeII lines. Results: Complementing the spectroscopic information with photometric transit curves and radial-velocity data from the literature, we have then refined the stellar and planetary radii and masses. The obtained data were also used to study and discuss the relation between the stellar metallicity and orbital period of the planets.

Odin Detection of O 2

We present the detection of molecular oxygen with Odin toward the dense molecular core rho Oph A, which is part of a region of active star formation. The observed spectral line is the (N,J = 1,1-1,0) ground state transition of molecular oxygen at 119 GHz (2.5 mm wavelength). The center of the line is at the LSR velocity of a number of optically thin lines from other species in the region and the O2 line also has a very similar, narrow, line width. Within the 10 arcmin beam, the integrated line intensity is 28 mK km/s, which corresponds to 5 sigma of the rms noise. A standard LTE analysis results in an O2 abundance of 5E(-8), with an uncertainty of at least a factor of two. We show that standard methods, however, do not apply in this case, as the coupling of the Odin beam to the source structure needs to be accounted for. Preliminary model results indicate O2 abundances to be higher by one order of magnitude than suggested by the standard case. This model predicts the 487 GHz line of O2 to be easily detectable by the future Herschel-HIFI facility, but to be out of reach for observations on a shorter time scale with the Odin space observatory.

Chemical abundances of 32 mildly metal-poor stars

Context. The formation scenario of the Galactic thick disk is an unresolved problem. Chemical abundances in long-lived dwarf stars of the thin and thick disks provide information of the Galactic disk formation. Aims. We present photospheric abundances of the O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Ni, and Ba elements for 32 mildly metal-poor stars with ${\rm [Fe/H]}\sim -0.7$. According to their kinematics, age, and [ α /Fe] , sample stars are identified to thin disk, thick disk, and halo population memberships. Element abundances for sample stars are discussed as a function of metallicity. Methods. High resolution and high signal-to-noise ratio spectra were obtained with the Coude Echelle Spectrograph mounted on the 2.16 m telescope at the National Astronomical Observatories (Xinglong, China). Effective temperatures were estimated from colour indices, and surface gravities from Hipparcos parallaxes. Stellar abundances were determined from a differential LTE analysis. The kinematics parameters were calculated from the parallax, proper motion, and radial velocity. Stellar ages were determined from theoretical stellar evolution tracks. Results. The average age of the thick disk stars is older than the thin disk stars. Our element abundance results extend and confirm previous works. The oxygen and other α -elements (Mg, Si, Ca, and Ti) abundances of thin and thick disk stars show distinct trends at $\rm [Fe/H]\le-0.60$. The [Al/Fe] behaviour is exactly as an α -element, although the separation for [Na/Fe] of thin and thick disk stars is not clear. The elements V, Cr, and Ni follow Fe very closely, and there is no offset between thin and thick disk stars, but the Sc and Mn abundance trends of the thin and thick disk stars are different, and [Ba/Fe] of thin disk and thick disk stars shows different behaviour.

An abundance analysis of the symbiotic star CH Cygni

Received; accepted Abstract. The photospheric abundances for the cool component of the symbiotic star CH Cyg were calculated for the first time using high-resolution near-infrared spectra and the method of of standard LTE analysis and atmospheric models. The iron abundance for CH Cyg was found to be solar, (Fe/H) = 0.0±0.19. The atmospheric parameters (Teff = 3100 K, log g = 0.0 (cgs),ξt = 2.2 km s −1 ) and metallicity for CH Cyg are found to be approximately equal to those for nearby field M7 giants. The calculated (C/H) = -0.15, (N/H) = +0.16, (O/H) = -0.07, and the isotopic ratios of 12 C/ 13 C and 16 O/ 17 O are close to the mean values for single M giants that have experienced the first dre dge-up. A reasonable explanation for the absence of barium star- like chemical peculiarities seems to be the high metallicit y of CH Cyg. The emission line technique was explored for estimating CNO ratios in the wind of the giant.

Quantitative spectroscopy of BA-type supergiants

Luminous BA-SGs allow topics ranging from NLTE physics and the evolution of massive stars to the chemical evolution of galaxies and cosmology to be addressed. A hybrid NLTE technique for the quantitative spectroscopy of BA-SGs is discussed. Thorough tests and first applications of the spectrum synthesis method are presented for four bright Galactic objects. Stellar parameters are derived from spectroscopic indicators. The internal accuracy of the method allows the 1sigma-uncertainties to be reduced to <1-2% in Teff and to 0.05-0.10dex in log g. Elemental abundances are determined for over 20 chemical species, with many of the astrophysically most interesting in NLTE. The NLTE computations reduce random errors and remove systematic trends in the analysis. Inappropriate LTE analyses tend to systematically underestimate iron group abundances and overestimate the light and alpha-process element abundances by up to factors of 2-3 on the mean. Contrary to common assumptions, significant NLTE abundance corrections of ~0.3dex can be found even for the weakest lines. NLTE abundance uncertainties amount to typically 0.05-0.10dex (random) and \~0.10dex (systematic 1sigma-errors). Near-solar abundances are derived for the heavier elements, and patterns indicative of mixing with nuclear-processed matter for the light elements. These imply a blue-loop scenario for Eta Leo, while the other three objects appear to have evolved directly from the main sequence. In the most ambitious computations several ten-thousand spectral lines are accounted for, permitting the accurate reproduction of the entire observed spectra from the visual to NIR. This prerequisite for the quantitative interpretation of medium-resolution spectra opens up BA-SGs as versatile tools for extragalactic stellar astronomy beyond the Local Group. (abridged)

Extreme helium stars: non-LTE matters

Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars - so-called extreme helium stars - to date face two major difficulties. First, theory fails to reproduce the observed helium lines in their entirety, wings and line cores. Second, a general mismatch exists for effective temperatures derived from ionization equilibria and from spectral energy distributions. Here, we demonstrate how the issue can be resolved using state-of-the-art non-LTE line-formation for these chemically peculiar objects. Two unique high-gravity B-type objects are discussed in detail, the pulsating variable V652 Her and the metal-poor star HD144941. In the first case atmospheric parameters from published LTE analyses are largely recovered, in the other a systematic offset is found. Hydrogen abundances are systematically smaller than previously reported, by up to a factor ~2. Extreme helium stars turn out to be important testbeds for non-LTE model atoms for helium. Improved non-LTE computations show that analyses assuming LTE or based on older non-LTE model atoms can predict equivalent widths, for the HeI 10830A transition in particular, in error by up to a factor ~3.

λ7774 Oxygen Triplet in Open Cluster Dwarfs: Pleiades and M34

We have undertaken a LTE analysis of the high-excitation 7774 A O I triplet in high-resolution, moderate signal-to-noise spectra of 15 Pleiades (HET/HRS) and 8 M34 (Keck/HIRES) open cluster dwarfs. Effective temperatures range from 5048 - 6172 K for the Pleiades sample and from 5290 - 6130 K for the M34 sample. Relative O abundances have been derived using model atmospheres interpolated from four different sets of ATLAS9 grids: with convective overshoot, without convective overshoot, with the mixing length parameter set to 0.5, and with the convective treatment of Canuto, Goldman, & Mazzitelli. In contrast to existing NLTE predictions, a dramatic increase in O I triplet abundance with decreasing temperature is seen for both clusters, regardless of atmospheric model. S I abundances of three Pleiads derived from the high-excitation λ 6052.67 feature mimic the O I abundance behavior. O abundances have also been derived from the 6300 A [OI] feature in three Pleiads; the abundances exhibit a much lower mean value than the cool dwarf triplet results. These data suggest LTE abundances derived from the O I triplet for cool dwarfs ( T eff ⩽ 5800 K ) should be viewed with caution.

CI non–LTE spectral line formation in late-type stars

We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non-LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al. (2004), the non-LTE abundance corrections are of the order of -0.35...-0.45 dex (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis.

The submillimeter C and CO lines in Henize 2-10 and NGC 253

The purpose of this paper is to describe a method for determining a cooling template for galaxies, using nearby galax- ies, and applicable to future observations of distant galaxies. We observed two starburst galaxies (NGC 253 and Henize 2-10) with the Caltech Submillimeter Observatory in the rotational lines of carbon monoxide 12 CO(J = 3-2), (J = 6-5) and (J = 7-6) for both, and also 12 CO(J = 4-3) and 13 CO(J = 3-2) for Henize 2-10 and in the 3 P2- 3 P1 fine-structure transitions of atomic carbon (CI) at 809 GHz for NGC 253. Some of these observations have been made previously, but the present multitransition study (including data found in the literature) is the most complete to date for the two galaxies. From these observations, we have derived the properties of the warm and dense molecular gas in the galaxy nuclei. We used an LTE analysis and an LVG radiative transfer model to determine physical conditions of the interstellar medium in both sources and predicted integrated line proper- ties of all CO transitions up to 12 CO(15-14). We found the observations to be in good agreement with a medium characterized by Tk ≈ 50-100 K, 12CO 13CO ≈ 30, n(H2) 10 4 cm −3 and N( 12 CO) = 3.5 ± 1 × 10 18 cm −2 for Henize 2-10 and characterized by Tk ≈ 70-150 K, 12CO 13CO ≈ 40, n(H2) 10 4 cm −3 and N( 12 CO) = 1.5 ± 0.5 × 10 19 cm −2 for NGC 253. A PDR model has also been used and here the data are well fitted (within 20%) by a model cloud with a gas density of n(H) = 8.0 ± 1 × 10 5 cm −3 and an incident FUV flux of χ ≈ 20 000 for Henize 2-10. For NGC 253, we deduced n(H) = 3.0 ± 0.5 × 10 5 cm −3 and χ ≈ 20 000 for the modelled cloud. The physical properties of warm gas and CO cooling curves of the target galaxies are compared with those measured for the nucleus of the Milky Way and the Cloverleaf QSO. The gas properties and CO cooling curve are similar for the two starburst galaxies and the Cloverleaf QSO while the Milky Way nucleus exhibits lower excitation molecular gas.

The submillimeter C and CO lines in Henize 2-10 and NGC 253

The purpose of this paper is to describe a method for determining a cooling template for galaxies, using nearby galaxies, and applicable to future observations of distant galaxies. We observed two starburst galaxies (NGC 253 and Henize 2-10) with the Caltech Submillimeter Observatory in the rotational lines of carbon monoxide 12CO(–2), (–5) and (–6) for both, and also 12CO(–3) and 13CO(–2) for Henize 2-10 and in the 3P2–3P1 fine-structure transitions of atomic carbon [CI] at 809 GHz for NGC 253. Some of these observations have been made previously, but the present multitransition study (including data found in the literature) is the most complete to date for the two galaxies. From these observations, we have derived the properties of the warm and dense molecular gas in the galaxy nuclei. We used an LTE analysis and an LVG radiative transfer model to determine physical conditions of the interstellar medium in both sources and predicted integrated line properties of all CO transitions up to 12CO(15–14). We found the observations to be in good agreement with a medium characterized by –100 K, , cm-3 and cm-2 for Henize 2-10 and characterized by –150 K, , cm-3 and cm-2 for NGC 253. A PDR model has also been used and here the data are well fitted (within 20%) by a model cloud with a gas density of cm-3 and an incident FUV flux of for Henize 2-10. For NGC 253, we deduced cm-3 and for the modelled cloud. The physical properties of warm gas and CO cooling curves of the target galaxies are compared with those measured for the nucleus of the Milky Way and the Cloverleaf QSO. The gas properties and CO cooling curve are similar for the two starburst galaxies and the Cloverleaf QSO while the Milky Way nucleus exhibits lower excitation molecular gas.

Magnesium Isotope Ratios in Hyades Stars

Using classical model atmospheres and an LTE analysis, Mg isotope ratios 24Mg:25Mg:26Mg are measured in 32 Hyades dwarfs covering 4000K < Teff < 5000K. We find no significant trend in any isotope ratio versus Teff and the mean isotope ratio is in excellent agreement with the solar value. We determine stellar parameters and Fe abundances for 56 Hyades dwarfs covering 4000K < Teff < 6200K. For stars warmer than 4700K, we derive a cluster mean value of [Fe/H] = 0.16 +/- 0.02 (sigma=0.1), in good agreement with previous studies. For stars cooler than 4700K, we find that the abundance of Fe from ionized lines exceeds the abundance of Fe from neutral lines. At 4700K [Fe/H]_II - [Fe/H]_I = 0.3 dex while at 4000K [Fe/H]_II - [Fe/H]_I = 1.2 dex. This discrepancy between the Fe abundance from neutral and ionized lines likely reflects inadequacies in the model atmospheres and the presence of Non-LTE or other effects. Despite the inability of the models to reproduce ionization equilibrium for Fe, the Mg isotope ratios appear immune to these problems and remain a powerful tool for studying Galactic chemical evolution.

A Survey of SiO 5 → 4 Emission toward Outflows from Low‐Luminosity Protostellar Candidates

We have observed the SiO J=5!4 line towards a sample of 25 low-luminosity (L∗ < 10 3 L⊙) protostellar outflow systems. The line was detected towards 7 of the 25 sources, a detection rate of 28 per cent. The majority (5 out of 7) of sources detected were of class 0 status. We detected a higher fraction of class 0 sources compared with the class I and II sources, although given the small numbers involved the significance of this result should be regarded as tentative. Most of the detected sources showed emission either at or close to the central position, coincident with the protostar. In four cases (HH211, HH25MMS, V-380 OriNE and HH212) emission was also detected at positions away from the center, and was stronger than that observed at the centre position. SiO abundances of 10 −8 to 8×10 −7 are derived from LTE analysis. For 2 sources we have additional transitions which we use to conduct statistical equilibrium modeling to estimate the gas density in the SiO-emitting regions. For HH25MMS these results suggest that the SiO emission arises in a higherdensity region than the methanol previously observed. We find that the most likely explanation for the preferential detection of SiO emission towards class 0 sources is the greater density of those environments, reinforced by higher shock velocities. We conclude that while not all class 0 sources exhibit SiO emission, SiO emission is a good signpost for the presence of class 0 sources. Subject headings: stars: formation — ISM: jets and outflows — ISM: molecules — ISM: Herbig-Haro objects — radio lines: ISM

The Photospheric Abundances of Active Binaries: First Results

Photospheric parameters and abundances of 13 chemical species are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (—0.41 ≲ [Fe/H] ≲ +0.11). A significant overabundance of several elements (e.g., Na, Mg, Al, Ca) is observed. The temperatures derived from the (V — R) and (V — I) colours are found to be significantly affected by activity processes.

The photospheric abundances of active binaries

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been derived for 13 chemical species, including several key elements such as Li, Mg, and Ca. Two methods have been used. The effective temperatures, surface gravities and microturbulent velocities were first derived from a fully self-consistent analysis of the spectra, whereby the temperature is determined from the excitation equilibrium of the lines. The second approach relies on temperatures derived from the () colour index. These two methods give broadly consistent results for the stars in our sample, suggesting that the neutral iron lines are formed under conditions close to LTE. We discuss the reliability in the context of chromospherically active stars of various colour indices used as temperature indicators, and conclude that the () and () colours are likely to be significantly affected by activity processes. Irrespective of the method used, our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (). A significant overabundance of several chemical species is observed (e.g., the α-synthezised elements). These abundance patterns are discussed in relation to stellar activity.

Oxygen Abundances in Metal‐poor Stars

We present oxygen abundances derived from both the permitted and forbidden oxygen lines for 55 subgiants and giants with [Fe/H] values between -2.7 and solar with the goal of understanding the discrepancy in the derived abundances. A first attempt, using Teff values from photometric calibrations and surface gravities from luminosities obtained agreement between the indicators for turn-off stars, but the disagreement was large for evolved stars. We find that the difference in the oxygen abundances derived from the permitted and forbidden lines is most strongly affected by Teff, and we derive a new Teff scale based on forcing the two sets of lines to give the same oxygen abundances. These new parameters, however, do not agree with other observables, such as theoretical isochrones or Balmer-line profile based Teff determinations. Our analysis finds that one-dimensional, LTE analyses (with published non-LTE corrections for the permitted lines) cannot fully resolve the disagreement in the two indicators without adopting a temperature scale that is incompatible with other temperature indicators. We also find no evidence of circumstellar emission in the forbidden lines, removing such emission as a possible cause for the discrepancy.

Physical properties of molecular clouds as functions of their Galactocentric distance from CS and C34S observations

Twenty-eight CS molecular clouds toward HII regions with Galactocentric distances from ∼ 4 to 20 kpc have been studied based on observations obtained in the J=2→1 lines of CS and C34S on the 20-meter radio telescope of the Onsala Space Observatory (Sweden) in March 2001. All 28 clouds have been mapped with an angular resolution of ∼40″. The peak intensity in the C34S line has been measured for 20 objects. An LTE analysis has been performed and the parameters of the molecular cloud cores derived. The core sizes are dA=0.3–4.8 pc, with a median value of ∼1.6 pc. The mean hydrogen densities in the cloud cores are nH2=3.5×102–3.7 × 104 cm−3, with a median value of ∼7.2×103 cm−3. The value of nH2 ends to decrease with increasing Galactocentric distance of the cloud. The masses of most clouds are 102−6×103M⊙, with the most probable value being MCS∼103M⊙. The data follow the dependence MCS ∝d A (2.4–3.2) . As a rule, the cloud masses are lower than the virial masses for MCS<103M⊙.

Chemical composition of B-type supergiants in the OB 8, OB 10, OB 48, OB 78 associations of M 31

Absolute and differential chemical abundances are presented for the largest group of massive stars in M 31 studied to date. These results were derived from intermediate resolution spectra of seven B-type supergiants, lying within four OB associations covering a galactocentric distance of 5-12 kpc. The results are mainly based on an LTE analysis, and we additionally present a full non-LTE, unified model atmosphere analysis of one star (OB 78-277) to demonstrate the reliability of the differential LTE technique. A comparison of the stellar oxygen abundance with that of previous nebular results shows that there is an offset of between ~ dex between the two methods which is critically dependent on the empirical calibration adopted for the R23 parameter with [O/H]. However within the typical errors of the stellar and nebular analyses (and given the strength of dependence of the nebular results on the calibration used) the oxygen abundances determined in each method are fairly consistent. We determine the radial oxygen abundance gradient from these stars, and do not detect any systematic gradient across this galactocentric range. We find that the inner regions of M 31 are not, as previously thought, very “metal rich”. Our abundances of C, N, O, Mg, Si, Al, S and Fe in the M 31 supergiants are very similar to those of massive stars in the solar neighbourhood.

A multiwavelength study of the S106 region

The molecular cloud associated with Sharpless 106 has been studied in a variety of (sub)millimeter CO rotational lines on angular resolution scales from 11 00 to 80 00 . We used the KOSMA 3 m telescope to obtain an extended 12 CO J =3 !2 map, from which we calculate a total mass of 2000 M and an average density of 1:4 10 3 cm 3 for the molecular cloud. The peak intensity region around the massive young star S106 IR was observed in 13 CO J =6 ! 5a nd 3! 2w ith KOSMA and in isotopomeric low-J CO lines with the IRAM 30 m telescope. A clump decomposition made for several lines yields a common clump-mass spectral index of =1 :7, illustrating the self-similarity of the detected structures for length-scales from 0.06 to 0.9 parsec. All 12 CO and 13 CO line proles within approximately 2 0 around S106 IR show blue wing emission and less prominent red wing emission, partly aected by self-absorption in colder foreground material. We attribute this high-velocity emission to the ionized wind of S106 IR driving a shock into the inhomogeneous molecular cloud. We do not nd evidence for a smooth or fragmented disk around S106 IR and/or an expanding ring in the observed CO emission distribution. The excitation conditions along a cut through the molecular cloud/H II region are studied with an LTE analysis (and an Escape Probability model at the position of S106 IR), using the observed CO line intensities and ratios. The kinetic gas temperature is typically 40 K, the average density of the cloud in the core region is 9 10 3 cm 3 , and the local density within the clumps is 9 10 4 cm 3 .T he 13 CO/C 18 O line and column density ratios away from S106 IR reflect the natural isotopic abundance but towards the optical lobes and the cavity walls, we see enhanced 13 CO emission and abundance with respect to C 18 O. This shows that selective photo-dissociation is only important close to S106 IR and in a thin layer of the cavity walls. In combination with the results from the excitation analysis we conclude that the molecular line emission arises from two dierent gas phases: (i) rather homogeneous, low- to medium-density, spatially extended clumps and (ii) embedded, small (0.2 pc), high-density clumps with a low volume lling factor.

Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis

We describe observations and abundance analysis of a high-resolution, high signal-to-noise ratio survey of 168 stars, most of which are metal-poor dwarfs. We follow a self-consistent LTE analysis technique to determine the stellar parameters and abundances, and we estimate the effects of random and systematic uncertainties on the resulting abundances. Element-to-iron ratios are derived for key α-, odd-Z, Fe-peak, and r- and s-process elements. Effects of non-LTE on the analysis of Fe I lines are shown to be very small on average. Spectroscopically determined surface gravities are derived that are quite close to those obtained from Hipparcos parallaxes.

High-resolution optical spectroscopy of the sharp-lined B-type star HD 83206

Very-high-resolution (R∼160 000) spectroscopic observations are presented for the early B-type star, HD 83206. Because it has very sharp metal lines, this star affords an opportunity to test theories of model atmospheres and line formation. Non-LTE model atmosphere calculations have been used to estimate the atmospheric parameters and absolute metal abundances (C, N, O, Mg and Si); an LTE analysis was also undertaken to investigate the validity of this simpler approach and to estimate an iron abundance. For the non-LTE calculations, there is excellent agreement with observations of the Balmer lines Hα and Hδ and the lines of Si ii and Si iii for atmospheric parameters of Teff≃21 700±600 K and log g≃4.00±0.15 dex. The agreement is less convincing for the LTE calculations, and a higher gravity is deduced. Careful comparison of the metal line profiles with non-LTE calculations implies that the projected rotational and microturbulent velocities have maximum values of ≃5 and ≃2 km s−1, respectively. The latter value is smaller than has often been adopted in LTE model atmosphere analyses of main-sequence stars. Non-LTE absolute metal abundances are estimated, and a comparison with those for normal B-type stars (deduced using similar non-LTE techniques) shows no significant differences. A comparison of the abundances deduced using non-LTE and LTE calculations implies systematic differences of 0.1–0.2 dex, showing the importance of using a non-LTE approach when accurate absolute abundances are required. Its location in the Hertzsprung–Russell diagram and normal metal abundance lead us to conclude that HD 83206 is probably a main-sequence B-type star. As such, it is among the sharpest-lined young B-type star discovered to date.