University College London Echelle Spectrograph Research Articles

Measurements of chromospheric Mg i emission lines of zero-age main-sequence stars

Abstract The chromosphere is the active atmosphere in which energetic eruption events, such as flares, occur. Chromospheric activity is driven by the magnetic field generated by stellar rotation and convection. The relationship between chromospheric activity and the Rossby number, the ratio of the rotational period to the convective turnover time, has been extensively examined for many types of stars, by using narrow chromospheric emission lines, such as the Ca ii lines and the Mg ii H and K lines. However, the stars with small Rossby numbers, i.e., stars with rapid rotations and/or long convective turnover times, show constant strengths of such lines against the Rossby number. In this study, we investigate the infrared Mg i emission lines at 8807 Å of 47 zero-age main-sequence (ZAMS) stars in IC 2391 and IC 2602 using the archive data of the Anglo-Australian Telescope at the University College London Echelle Spectrograph. After subtracting the photospheric absorption component, the Mg i line is detected as an emission line for 45 ZAMS stars, the equivalent widths of which are between 0.02 Å and 0.52 Å. A total of 42 ZAMS stars show the narrower Mg i emission lines instead of the Ca ii infrared triplet emission lines, suggesting that they are formed at different depths. The ZAMS stars with smaller Rossby numbers show stronger Mg i emission lines. The Mg i emission line is not saturated even in the saturated regime of the Ca ii emission lines, i.e., Rossby number <10−1.1. The Mg i emission line is considered to be a good indicator of chromospheric activity, particularly for active objects.

Rotation-Activity Relation of Ca II and Mg I Infrared Emission Lines of Young Stars

To reveal detail of internal structure, relationship between chromospheric activity and Rossby number, N_R (= rotational period P / convective turnover time tau_c), has been extensively examined for main-sequence stars. The goal of our work is to apply same methods to pre-main-sequence (PMS) stars and identify appropriate model of tau_c for them. Yamashita et al. (2020) investigated relationship between N_R and strengths of Ca II infrared triplet (IRT; lambda 8498, 8542, 8662 A) emission lines of 60 PMS stars. Their equivalent widths are converted into emission line to stellar bolometric luminosity ratio (R'). The 54 PMS stars have N_R < 10^{-1.0} and show R' \sim 10^{-4.2} as large as maximum R' of zero-age main-sequence (ZAMS) stars. However, because all R' was saturated against N_R, it was not possible to estimate appropriate tau_c model for PMS stars. We noticed that Mg I emission lines at 8808 A is an optically thin chromospheric line, appropriate for determination of adequate tau_c for PMS stars. Using archive data of Anglo-Australian Telescope (AAT)/the University College London Echelle Spectrograph (UCLES), we investigated Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, Mg I line is detected as an emission line in 45 ZAMS stars, whose R' is between 10^{-5.9} and 10^{-4.1}. The Mg I line is not saturated yet in the saturated regime for Ca II emission lines, i.e. 10^{-1.6} < N_R < 10^{-0.8}. Therefore, adequate tau_c for PMS stars can be determined by measuring of their R' values.

Deriving Iodine-free Spectra for High-resolution Echelle Spectrographs*

Abstract We describe a new method to derive clean, iodine-free spectra directly from observations acquired using high-resolution echelle spectrographs equipped with iodine cells. The main motivation to obtain iodine-free spectra is to use portions of the spectrum that are superimposed with the dense forest of iodine absorption lines, in order to retrieve lines that can be used to monitor the magnetic activity of the star, helping to validate candidate planets. In short, we provide a straightforward methodology to clean the spectra using the forward model used to derive radial velocities, the line spread function information plus the stellar spectrum without iodine to reconstruct and subtract the iodine spectrum from the observations. We show our results using observations of the star τ Ceti acquired with the Planet Finder Spectrograph (PFS), High Resolution Echelle Spectrometer (HIRES), and University College London Echelle Spectrograph (UCLES), reaching an iodine-free spectrum correction at the ∼1% rms level. We additionally discuss the limitations and further applications of the method.

A young solar twin in the Rosette cluster NGC 2244 line of sight

Based on prior precision photometry and cluster age analysis, the bright star GSC 00154−01819 is a possible young pre-main sequence member of the Rosette cluster, NGC 2244. As part of a comprehensive study of the large-scale structure of the Rosette and its excitation by the cluster stars, we noted this star as a potential backlight for a probe of the interstellar medium and extinction along the sight line towards a distinctive nebular feature projected on to the cluster centre. New high-resolution spectra of the star were taken with the University College London Echelle Spectrograph of the AAT. They reveal that rather than being a reddened spectral type B or A star within the Mon OB2 association, it is a nearby, largely unreddened, solar twin of spectral type G2V less than 180 Myr old. It is about 219 pc from the Sun with a barycentric radial velocity of +14.35 ± 1.99 km s−1. The spectrum of the Rosette behind it and along this line of sight shows a barycentric radial velocity of +26.0 ± 2.4 km s−1 in H α, and a full width at half-maximum velocity dispersion of 61.94 ± 1.38 km s−1.

SMALL-SCALE STRUCTURE OF THE INTERSTELLAR MEDIUM TOWARD ρ Oph STARS: DIFFUSE BAND OBSERVATIONS

We present an investigation of small-scale-structure in the distribution of large molecules/dust in the interstellar medium through observations of diffuse interstellar bands (DIBs). High signal-to-noise optical spectra were recorded towards the stars rho Oph A, B, C and DE using the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope. The strengths of some of the DIBs are found to differ by about 5-9 % between the close binary stars rho Oph A and B, which are separated by a projected distance on the sky of only ~344 AU. This is the first star-system in which such small-scale DIB strength variations have been reported. The observed variations are attributed to differences between a combination of carrier abundance and the physical conditions present along each sightline. The sightline towards rho Oph C contains relatively dense, molecule-rich material and has the strongest \lambda\lambda 5850 and 4726 DIBs. The gas towards DE is more diffuse and is found to exhibit weak C_2 (blue) DIBs and strong yellow/red DIBs. The differences in diffuse band strengths between lines of sight are, in some cases, significantly greater in magnitude than the corresponding variations among atomic and diatomic species, indicating that the DIBs can be sensitive tracers of interstellar cloud conditions.

The chromospheric emission of solar-type stars in the young open clusters IC 2391 and IC 2602

In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High resolution spectroscopic data were obtained for over 50 F, G, and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid-rotators (vsini > 20 km/s) with five of the stars being classified as ultra-rapid rotators (vsini > 100 km/s). The emission levels in the Calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against Rossby number (NR) the star's chromospheric emission levels show a plateau in the emission for Log(NR) < -1.1 indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars show little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators.

Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue ��� I. A sample of systems with components' masses between 1 and 2 M���

We derive the absolute physical and orbital parameters for a sample of 18 detached eclipsing binaries from the \emph{All Sky Automated Survey} (ASAS) database based on the available photometry and our own radial velocity measurements. The radial velocities (RVs) are computed using spectra we collected with the 3.9-m Anglo-Australian Telescope and its \emph{University College London Echelle Spectrograph} and the 1.9-m SAAO Radcliffe telescope and its \emph{Grating Instrument for Radiation Analysis with a Fibre Fed Echelle}. In order to obtain as precise RVs as possible, most of the systems were observed with an iodine cell available at the AAT/UCLES and/or analyzed using the two-dimensional cross-correlation technique (TODCOR). The RVs were measured with TODCOR using synthetic template spectra as references. However, for two objects we used our own approach to the tomographic disentangling of the binary spectra to provide observed template spectra for the RV measurements and to improve the RV precision even more. For one of these binaries, AI Phe, we were able to the obtain an orbital solution with an RV $rms$ of 62 and 24 m s$^{-1}$ for the primary and secondary respectively. For this system, the precision in $M \sin^3{i}$ is 0.08%. For the analysis, we used the photometry available in the ASAS database. We combined the RV and light curves using PHOEBE and JKTEBOP codes to obtain the absolute physical parameters of the systems. Having precise RVs we were able to reach $\sim$0.2 % precision (or better) in masses in several cases but in radii, due to the limited precision of the ASAS photometry, we were able to reach a precision of only 1% in one case and 3-5 % in a few more cases. For the majority of our objects, the orbital and physical analysis is presented for the first time.

Doppler images and chromospheric variability of TWA 17

We present Doppler imaging and a Balmer line analysis of the weak-line T Tauri star TWA 17. Spectra were taken in 2006 with the University College London Echelle Spectrograph on the Anglo-Australian Telescope. Using least-squares deconvolution to improve the effective signal-to-noise ratio, we produced a Doppler map of the surface spot distribution. This shows similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar spot with more spots extending out of it down to the equator. In addition to the photospheric variability, the chromospheric variability was studied using the Balmer emission. The mean Hα profile has a narrow component consistent with rotational broadening and a broad component extending out to 220 km s−1. The variability in Hα suggests that the chromosphere has at least one slingshot prominence 3R* above the surface.

Young Star Clusters in the Small Magellanic Cloud: Impact of Local and Global Conditions on Star Formation

AbstractWe compared deep images acquired with the Advanced Camera for Surveys on board of the Hubble Space Telescope with mid-IR Spitzer Space Telescope images and University College London Echelle Spectrograph spectra of NGC 346 and NGC 602, two of the youngest star clusters in the Small Magellanic Cloud. Our multi-wavelength approach allowed us to infer very different origins for the clusters: while NGC 346 is likely the result of the hierarchical collapse of a giant molecular cloud, NGC 602 is probably the result of the collision and consequent interaction of two H I shells of gas.

A non-LTE abundance analysis of the post-AGB star ROA 5701

An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet (UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is performed, using non-local thermodynamic equilibrium (non-LTE) model atmospheres to estimate stellar atmospheric parameters and chemical composition. Abundances are derived for C, N, O, Mg, Si and S, and compared with those found previously by Moehler et al. We find a general metal underabundance relative to young B-type stars, consistent with the average metallicity of the cluster. Our results indicate that ROA 5701 has not undergone a gas-dust separation scenario as previously suggested. However, its abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the onset of the third dredge-up.

Doppler imaging and surface differential rotation of young open cluster stars - I. HD 307938 (R58) in IC 2602

In this paper we present Doppler images of a young active G dwarf (HD 307938) in the southern open cluster IC 2602. Spectroscopic data were obtained over a four-night period in 2000 January at the 3.9-m Anglo-Australian Telescope using the University College London Echelle Spectrograph. Simultaneous photometric observations (in the V and R bands) were obtained at the 1.0-m Australian National University telescope. By applying least-squares deconvolution (LSD) to the 2500+ photospheric lines in each echelle spectrum a single high signal-to-noise ratio LSD profile was produced for each phase of the spectroscopic observations. Maximum-entropy image reconstruction, incorporating both the LSD profiles and the photometric data, was used to produce maps of the surface features of the star, with the inclusion of the photometric data producing an increase (compared with the use of spectroscopic data alone) in the spot occupancy in both low- and mid-latitude regions of the star. The maps show that HD 307938 possesses a large, broken polar spot extending down to ∼60° latitude, as well as lower-latitude spots similar to other rapidly rotating G dwarfs. By incorporating a solar-like differential rotation law into the imaging process the surface differential rotation of HD 307938 was determined. This gave a surface shear of dΩ= 0.025 ± 0.015 rad d−1 (for an inclination angle of 60°). Thus the equator of HD 307938 laps the poles every ∼250 d and has a photospheric shear around half that of the Sun.

Emission Lines of [Clii] in the Optical Spectra of Gaseous Nebulae

Recent R-matrix calculations of electron impact excitation rates among the 3s23p4 levels of Cl II are used to derive the nebular emission-line intensity ratios R1 = I(6161.8 A)/I(8578.7 A) and R2 = I(6161.8 A)/I(9123.6 A) as a function of electron temperature (Te) and density (Ne). The ratios are found to be very sensitive to changes in Te but not Ne for densities lower than 105 cm-3. Hence, they should, in principle, provide excellent optical Te diagnostics for planetary nebulae. The observed values of R1 and R2 for the planetary nebulae NGC 6741 and IC 5117, measured from spectra obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope, imply temperatures in excellent agreement with those derived from other diagnostic lines formed in the same region of the nebula as [Cl II]. This provides some observational support for the accuracy of the [Cl II] line ratio calculations and hence the atomic data on which they are based. The [Cl II] 8578.7 and 9123.6 A lines are identified for the first time (to our knowledge) in a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope. However, the 6161.8 A feature is unfortunately too weak to be identified in the RR Telescopii observations, consistent with its predicted line strength.

Emission lines of [K V] in the optical spectra of gaseous nebulae.

Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I(4122.6 A)/I(4163.3 A) as a function of electron density (N(e)). This ratio is found to be very sensitive to changes in N(e) over the density range 10(3) to 10(6) cm(-3), but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical N(e) diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne iv], formed in the same region of the nebula as [K v]. This observation provides observational support for the accuracy of the theoretical [K v] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope, reveals that the [K v] 4122.6 A line in this object is badly blended with Fe ii 4122.6 A. Hence, the [K v] diagnostic may not be used for astrophysical sources that show a strong Fe ii emission line spectrum.

Line identifications and intensities for the optical spectrum of RR Telescopii between 3180 and 9455 Å

The symbiotic nova RR Telescopii has been ob- served with the 3.9 m telescope at the Anglo-Australian Observatory (AAO), using the University College London Echelle Spectrograph (UCLES) in conjunction with a Tek CCD. It displays a rich emission line spectrum, ranging in excitation from N i to (Ni viii). We present a list of 811 measured lines, with their suggested identications and absolute line intensities, covering a wavelength range from 3180 A to 9455 A 1 . The absolute line intensities have been derived by comparing the high resolution data with a flux-calibrated low resolution spectrum taken with the Australian National University 2.3 m telescope. All of the lines have been successfully identied. Comparing our re- sults with those of previous studies indicates that the RR Tel system is advancing towards higher degrees of ionisation.

Echelle spectrograph observations of the Hubble Deep Field South QSO J2233--606

We report on high-resolution observations (R=35 000) of the Hubble Deep Field South QSO J2233-606 obtained with the University College London Echelle Spectrograph at the Anglo-Australian Telescope. We present spectral data for the wavelength region 3530 3700 A. Voigt profiles are fitted to determine the redshift and column densities of the observed features. Eight heavy-element intervening systems are identified, as well as an associated system showing evidence for incomplete coverage of the background source.

High-resolution interstellar and stellar line profiles of stars in the globular cluster M22

Summary High-resolution (˜5 km s−1 FWHM) spectra have been obtained for stars in the globular cluster M22 using the University College London echelle spectrograph (UCLES) at the coudé focus of the AAT. The prime objective was a study of the fine-scale spatial structure of the line-of-sight gas through observations of the Na D lines. Essentially the gas towards the cluster at distance ˜ 6 kpc is concentrated in two components at the approximate velocities (LSR) of + 6 and + 37 km s−1. The lower velocity material is likely to be part of a nearby H I cloud, first mapped in the radio region, in which case the spatial sampling in the present study has a scale &amp;lt; 0.3 pc. The higher velocity gas varies from a single component to a multiple component structure over the angular extent of the cluster. The distance to this material is less certain but it may be associated with the Sagittarius arm. New observations are presented also for the stellar Hα and Na D lines. Time variability of Hα emission and Na D line asymmetry are detected for two stars. The present data are in qualitative agreement with models for mass loss occurring in expanding, circumstellar envelopes.