H. Hensberge

HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope

The HERMES high-resolution spectrograph project aims at exploiting the specific potential of small but flexible telescopes in observational astrophysics. The optimised optical design of the spectrograph is based on the well-proven concept of white-pupil beam folding for high-resolution spectroscopy. In this contribution we present the complete project, including the spectrograph design and procurement details, the telescope adaptor and calibration unit, the detector system, as well as the optimised data-reduction pipeline. We present a detailed performance analysis to show that the spectrograph performs as specified both in optical quality and in total efficiency. With a spectral resolution of 85 000 (63 000 for the low-resolution fibre), a spectral coverage from 377 to 900 nm in a single exposure and a peak efficiency of 28%, HERMES proves to be an ideal instrument for building up time series of high-quality data of variable (stellar) phenomena.

Chemically tagging the Hyades stream: does it partly originate from the Hyades cluster?

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the actionspace distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metallicity as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars (namely 2/21) from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of mainsequence stars is very efficient in the range 5500 K ≤ T eff ≤ 6200 K, since the Li sequence in the Hyades cluster is very tight, while at the same time spanning a large abundance range; (iv) we show that, while this evaporated population has a metallicity excess of ∼0.2 dex with respect to the local thin-disc population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ∼0.06–0.15 dex, consistent with an origin in the inner Galaxy and (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

Abundances from disentangled component spectra: the eclipsing binary V578 Mon

Chemical abundances of the early-B type components of the binary V578 Mon are derived from disentangled component spectra. This is a pilot study showing that, even with moderately high line-broadening, metal abundances can be derived from disentangled spectra with a precision of 0.1 dex, differential to sharp-lined single stars of the same spectral type. This binary is well-suited to such an assessment because of its youth as a member of the Rosette Nebula cluster NGC 2244, strengthening the expectation of an unevolved ZAMS chemical composition. The method is useful for studying rotationally driven mixing in main-sequence stars, since fundamental stellar parameters are known with higher accuracy in (eclipsing) binaries. This paper also evaluates of the bias that might be present in disentangled spectra.

Mass ratio from Doppler beaming and Rømer delay versus ellipsoidal modulation in the Kepler data of KOI-74

We present a light curve analysis and radial velocity study of KOI-74, an eclipsing A star\(+\) white dwarf binary with a 5.2 day orbit. Aside from new spectroscopy covering the orbit of the system, we used 212 days of publicly available Kepler observations and present the first complete light curve fitting to these data, modelling the eclipses and transits, ellipsoidal modulation, reflection, and Doppler beaming. Markov Chain Monte Carlo simulations are used to determine the system parameters and uncertainty estimates. Our results are in agreement with earlier studies, except that we find an inclination of \(87.0\pm 0.4^\circ \), which is significantly lower than the previously published value. The altered inclination leads to different values for the relative radii of the two stars and therefore also the mass ratio deduced from the ellipsoidal modulations seen in this system. We find that the mass ratio derived from the radial velocity amplitude (\(q=0.104\pm 0.004\)) disagrees with that derived from the ellipsoidal modulation (\(q=0.052\pm 0.004\) assuming corotation). This mismatch was found before, but with our smaller inclination, the discrepancy is even larger than previously reported. Accounting for the rapid rotation of the A-star, instead of assuming corotation with the binary orbit, is found to increase the discrepancy even further by lowering the mass ratio to \(q=0.047\pm 0.004\). These results indicate that one has to be extremely careful in using the amplitude of an ellipsoidal modulation signal in a close binary to determine the mass ratio, when a proof of corotation is not firmly established. The same problem could arise whenever an ellipsoidal modulation amplitude is used to derive the mass of a planet orbiting a host star that is not in corotation with the planet’s orbit. The radial velocities that can be inferred from the detected Doppler beaming in the light curve are found to be in agreement with our spectroscopic radial velocity determination. We also report the first measurement of Romer delay in a light curve of a compact binary. This delay amounts to \(-56\pm 17\) s and is consistent with the mass ratio derived from the radial velocity amplitude. The firm establishment of this mass ratio at \(q=0.104\pm 0.004\) leaves little doubt that the companion of KOI-74 is a low mass white dwarf.

On the separation of component spectra in binary and higher-multiplicity stellar systems: bias progression and spurious patterns

Context. Powerful methods are available to reconstruct the spectra of stars in orbit around each other, using a time-series of observed, composite spectra. They act either on the Fourier components of the observed spectra, or directly on these spectra in a velocity grid. Aims. We discuss under which conditions spurious patterns can appear in reconstructed spectra, either as a consequence of (quasi)degeneracy of the equations or as a consequence of bias in the observed spectra. Also we discuss the equivalence of Fourier and direct methods in practice. Methods. We show under which conditions the equations degenerate, and how to evaluate this. We pay special attention to spectra of binary stars and triple systems analysed in Fourier space. We apply the theory to real data sets and to artificial data sets with several types of data-reduction bias constructed to illustrate degeneracies and the transfer of bias to the reconstructed spectra. Results. Quasi-degeneracy of the equations depends on the lack of significant time-variability in the relative light contribution of the stars, on the length of the spectral interval in units of the involved Doppler shifts, on the presence of very faint stellar components, and on the distribution of the observations over the orbital phases. Eclipse spectra, possibly used with different weight in low- and highfrequency Fourier modes, remove quasi-degeneracies. But when the normalisation of the observed spectra is biased in a systematic way with relation to the orbital phase, then the bias amplifies strongly in the reconstructed spectra, particularly for the faintest components. Wavelength-locked bias is transferred more strongly to the spectrum of the star with the lowest velocity amplitude. Unrecognized variations in line strength lead to bias that is larger in spectral regions with high line-density. Most importantly, the bias in all reconstructed spectra is coupled in a unique, predictable way. This coupling allows us to design robust procedures for the removal of bias from the reconstructed spectra. Conclusions. When the spectral features of all stars visible in the observed spectra undergo a significant time-variable dilution effect, then the reconstructed spectra are well-defined. Otherwise, spurious patterns, mostly of low frequency, may be superimposed on the reconstructed spectra. The analysis presented in this paper allows observers to optimise their observing strategy, gives insight in the origin of spurious patterns and indicates ways either to suppress such patterns or to remove them a posteriori from the reconstructed spectra. In this way, a broader range of astrophysical analyses can be applied to the component spectra.

The evolutionary state of Miras with changing pulsation periods

Context. Miras are long-period variables thought to be in the asymptotic giant branch (AGB) phase of evolution. In about one percent of known Miras, the pulsation period is changing. It has been speculated that this changing period is the consequence of a recent thermal pulse in these stars. Aims. We aim to clarify the evolutionary state of these stars, and to determine in particular whether or not they are in the thermallypulsing (TP-)AGB phase. Methods. One important piece of information that has been neglected so far when determining the evolutionary state is the presence of the radio-active s-process element technetium (Tc). We obtained high-resolution, high signal-to-noise-ratio optical spectra of a dozen prominent Mira variables with changing pulsation period to search for this indicator of TPs and dredge-up. We also use the spectra to measure lithium (Li) abundances. Furthermore, we establish the evolutionary states of our sample stars by means of their present-day periods and luminosities. Results. Among the twelve sample stars observed in this programme, five were found to show absorption lines of Tc. BH Cru is found to be a carbon-star, its period increase in the past decades possibly having stopped by now. We report a possible switch in the pulsation mode of T UMi from Mira-like to semi-regular variability in the past two years. R Nor, on the other hand, is probably a fairly massive AGB star, which could be true for all meandering Miras. Finally, we assign RU Vul to the metal-poor thick disk with properties very similar to the short-period, metal-poor Miras. Conclusions. We conclude that there is no clear correlation between period change class and Tc presence. The stars that are most likely to have experienced a recent TP are BH Cru and R Hya, although their rates of period change are quite different.

Spectral disentangling of the triple system DG Leo: orbits and chemical composition

DG Leo is a spectroscopic triple system composed of three stars of late-A spectral type, one of which was suggested to be a δ Scuti star. Seven nights of observations at high spectral and high time-resolution at the Observatoire de Haute-Provence with the ELODIE spectrograph were used to obtain the component spectra by applying a Fourier transform spectral disentangling technique. Comparing these with synthetic spectra, the stellar fundamental parameters (effective temperature, surface gravity, projected rotation velocity and chemical composition) are derived. The inner binary consists of two Am components, at least one of which is not yet rotating synchronously at the orbital period though the orbit is a circular one. The distant third component is confirmed to be a δ Scuti star with normal chemical composition.

Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster

During a survey of field strengths in upper main sequence stars in open clusters, we observed the star NGC 2244-334 in the Rosette Nebula cluster and discovered an extraordinarily large mean longitudinal field of about −9 kG, the second largest longitudinal field known in a non-degenerate star. This star appears to be a typical Ap He-wk (Si) star of about 4 M� . Spectrum synthesis using a line synthesis code incorporating the effects of the strong magnetic field indicates that He is underabundant by about 1.5 dex, and C, O and Mg by about 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex, and Cr and Ti are nearly 2 dex overabundant. Cluster membership for this star is secure, so its age is about 2 × 10 6 yr, which is less than 3% of its main sequence lifetime. This star is one of the very youngest magnetic upper main sequence stars with a well-determined age, and confirms that both magnetic fields and strong chemical peculiarity can appear in stars which are both extremely young and very close to the ZAMS.

Time-resolved spectroscopy of BD+46°442: Gas streams and jet creation in a newly discovered evolved binary with a disk

Context. Previous studies have shown that many post-asymptotic giant branch (AGB) stars with dusty disks are associated with single-lined binary stars. The inferred orbital separations are too small to accommodate a fully grown AGB star, hence these systems represent a new evolutionary channel that bypasses a full AGB evolution. Aims. We wish to verify the binarity hypothesis for a larger sample establish the nature of the companions, and probe the disk structure and eventually the disk formation mechanisms in binary stars. To achieve these aims, we started a high-resolution spectral monitoring of ~40 field giants whose binarity had been suspected based on either a light curve, an infrared excess, or a peculiar chemical composition. Methods. Starting from the spring of 2009, we monitored the programme stars with the fibre echelle spectrometer HERMES. We measure their radial velocities (RVs) with a precision of ~0.2 km s -1, perform detailed photospheric abundance analyses, and analyse the time-resolved high-resolution spectra to search for line-profile variability. Results. Here we report on the discovery of periodic RV variations in BD+46°442, a high Galactic latitude F giant with a disk. We infer that the variations are caused by the motion around a faint companion, and deduce the orbital parameters P orb = 140.77 ± 0.02 d,e = 0.083 ± 0.002, and asini = 0.31 AU. We find that it is a moderately metal-poor star ([M/H] =-0.7) without a strong depletion pattern in its photospheric abundances. Interestingly, many lines indeed show periodic changes with the orbital phase: Hα switches between a double-peak emission line and a P Cyg-like profile, while strong metal lines appear to be split at the maximum redshift. Similar effects are likely visible in the spectra of other post-AGB binaries, but their regularity is not always apparent owing to sporadic observations. We propose that these features result from an ongoing mass transfer from the evolved giant to the companion. In particular, the blue-shifted absorption in Hα, which occurs only at superior conjunction, may result from a jet originating in the accretion disk around the companion and that is seen in absorption towards the luminous primary.

The massive binary CPD 41 7742 I. High-resolution optical spectroscopy ?

We present the results of a spectroscopic campaign on the early-type binary CPD 41 7742. For the first time, we unambiguously detect the secondarys spectral signature and derive an accurate orbital solution for both components of the system. We confirm that the orbit displays a slight but definite eccentricity ( e = 0:027 0:006) despite the short period (P= 2:440700:00043 days). Previous radial velocity measurements available in the literature constitute together with our new observations a data set that spans more than 30 years. The combined primary orbital solution inferred is in excellent agreement with our solution and gives a period P= 2:44062 0:00005 days. Based on spectroscopic criteria, we derive a spectral and luminosity classification of O9 III + B1 III. However, the luminosities and radii inferred from the membership of NGC 6231 rather indicate lower luminosity classes. We show that the equivalent widths of well isolated primary lines display variations that suggest that CPD 41 7742 is an eclipsing binary. This makes CPD 41 7742 the second known SB2 eclipsing early-type binary of the NGC 6231 cluster. We approximately constrain the inclination of the system imin 60. This may indicate that the system does not oer enough room for two stars with radii typical of giant stars and lends further support to a less evolved luminosity classification for at least one of the objects.