Terminal-age Main Sequence Research Articles

NR Peg: A new highly active semi-detached binary

This paper presents the first analysis of spectroscopic and photometric observations of the eclipsing binary star NR Peg. ELODIE Hα observations indicate that the secondary component is a chromospherically active star; however, the spectral line profiles (especially of neutral metals) of NR Peg are very wide and have a complex structure with asymmetric bubbles on its branches, which could be interpreted as traces of stellar magnetic activity in both components. The 2007 and 2008 BVR light curves are generally those of β Lyrae-type eclipsing binaries, however, there are large asymmetries between maxima. We explained these peculiar asymmetries in terms of large dark spots on the primary component. ELODIE spectroscopic data and 2008 BVR light curves were solved simultaneously using Wilson–Devinney code. The results describe NR Peg as a RS CVn type binary star with a semi-detached configuration. The masses of the components were found to be 1.60±0.03 M⊙ and 0.57±0.02 M⊙ and the radii to be 3.35±0.07 R⊙ and 3.55±0.08 R⊙ for the primary and secondary components, respectively. Both components of NR Peg appear to have evolved behind the terminal age main sequence. However, the less-massive secondary component is significantly oversized and overluminous compared to theoretical evolution models. The distance of NR Peg was calculated as 138±12pc, taking into account interstellar extinction, in agreement with the HIPPARCOS value.

The observational signatures of convectively excited gravity modes in main-sequence stars

We predict the flux and surface velocity perturbations produced by convectively excited gravity modes (g-modes) in main sequence stars. Core convection in massive stars can excite g-modes to sufficient amplitudes to be detectable with high precision photometry by Kepler and CoRoT, if the thickness of the convective overshoot region is < 30 per cent of a pressure scale height. The g-modes manifest as excess photometric variability, with amplitudes of ~ 10 micromagnitudes at frequencies ~ 10 microHz (0.8 1/d) near the solar metallicity zero-age main sequence. The flux variations are largest for stars with M > 5 solar masses, but are potentially detectable down to M ~ 2 - 3 solar masses. During the main sequence evolution, radiative damping decreases such that ever lower frequency modes reach the stellar surface and flux perturbations reach up to ~ 100 micromagnitudes at the terminal-age main sequence. Using the same convective excitation model, we confirm previous predictions that solar g-modes produce surface velocity perturbations of < 0.3 mm/s. This implies that stochastically excited g-modes are more easily detectable in the photometry of massive main sequence stars than in the Sun.

A photometric study of chemically peculiar stars with the STEREO satellites – II. Non-magnetic chemically peculiar stars★

We have analysed the photometric data obtained with the STEREO spacecraft for 558 non-magnetic chemically peculiar (CP) stars to search for rotational and pulsational variability. Applying the Lomb–Scargle and the phase dispersion minimization methods, we have detected photometric variability for 44 objects from which 35 were previously unknown. The new objects are all bright stars on the ecliptic plane (magnitude range 4.7 < V < 11.7) and will therefore be of great interest to studies of stellar structure and evolution. In particular, several show multiple signals consistent with hybrid δ Scuti and γ Doradus pulsation, with different periodicities allowing very different regions of the stellar interior to be studied. There are two subgroups of stars in our sample: the cool metallic line Am (CP1) and the hot HgMn (CP3) stars. These objects fall well inside the classical instability strip where δ Scuti, γ Doradus and slowly pulsating B-type stars are located. We also expect to find periods correlated to the orbital period for CP1 objects as they are mostly members of binary systems. For CP3 stars, rotationally induced variability is still a matter of debate. Although surface spots were detected, they are believed to produce only marginal photometric amplitudes. So, periods from several hours to a few days were expected for these two star groups. The STEREO/HI-1 data are well matched to studies of this frequency domain, owing to the cadence of approximately 40 min and multiple epochs over four and a half years. The remaining 514 stars are likely to be constant in the investigated range from 0.1 to 10 d. In some cases, the presence of blending or systematic effects prevented us from detecting any reliable variability and in those cases we classified the star as constant. We discuss our results in comparison to already published ones and find a very good agreement. Finally, we have calibrated the variable stars in terms of the effective temperature and luminosity in order to estimate masses and ages. For this purpose, we used specifically developed calibrations for CP stars and, when available, Hipparcos parallaxes. All but two objects cover the stellar mass range from 1.5 to 5 M⊙ and are located between the zero- and terminal-age main sequence.

KOI 1224: A FOURTH BLOATED HOT WHITE DWARF COMPANION FOUND WITHKEPLER

We present an analysis and interpretation of the Kepler binary system KOI 1224. This is the fourth binary found with Kepler that consists of a thermally bloated, hot white dwarf in a close orbit with a more or less normal star of spectral class A or F. As we show, KOI 1224 contains a white dwarf with Teff = 14400 +/- 1100 K, mass = 0.20 +/- 0.02 Msun, and radius = 0.103 +/- 0.004 Rsun, and an F-star companion of mass = 1.59 +/- 0.07 Msun that is somewhat beyond its terminal-age main sequence. The orbital period is quite short at 2.69802 days. The ingredients that are used in the analysis are the Kepler binary light curve, including the detection of the Doppler boosting effect; the NUV and FUV fluxes from the Galex images of this object; an estimate of the spectral type of the F-star companion; and evolutionary models of the companion designed to match its effective temperature and mean density. The light curve is modelled with a new code named Icarus which we describe in detail. Its features include the full treatment of orbital phase-resolved spectroscopy, Doppler boosting, irradiation effects and transits/eclipses, which are particularly suited to irradiated eclipsing binaries. We interpret the KOI 1224 system in terms of its likely evolutionary history. We infer that this type of system, containing a bloated hot white dwarf, is the direct descendant of an Algol-type binary. In spite of this basic understanding of the origin of KOI 1224, we discuss a number of problems associated with producing this type of system with this short of an short orbital period.

The first evidence for multiple pulsation axes: a new rapidly oscillating Ap star in the Kepler field, KIC 10195926

We have discovered a new rapidly oscillating Ap star among the Kepler Mission target stars, KIC 10195926. This star shows two pulsation modes with periods that are amongst the longest known for roAp stars at 17.1 min and 18.1 min, indicating that the star is near the terminal age main sequence. The principal pulsation mode is an oblique dipole mode that shows a rotationally split frequency septuplet that provides information on the geometry of the mode. The secondary mode also appears to be a dipole mode with a rotationally split triplet, but we are able to show within the improved oblique pulsator model that these two modes cannot have the same axis of pulsation. This is the first time for any pulsating star that evidence has been found for separate pulsation axes for different modes. The two modes are separated in frequency by 55 microHz, which we model as the large separation. The star is an alpha^2 CVn spotted magnetic variable that shows a complex rotational light variation with a period of Prot = 5.68459 d. For the first time for any spotted magnetic star of the upper main sequence, we find clear evidence of light variation with a period of twice the rotation period; i.e. a subharmonic frequency of $\nu_{\rm rot}/2$. We propose that this and other subharmonics are the first observed manifestation of torsional modes in an roAp star. From high resolution spectra we determine Teff = 7400 K, log g = 3.6 and v sin i = 21 km/s. We have found a magnetic pulsation model with fundamental parameters close to these values that reproduces the rotational variations of the two obliquely pulsating modes with different pulsation axes. The star shows overabundances of the rare earth elements, but these are not as extreme as most other roAp stars. The spectrum is variable with rotation, indicating surface abundance patches.

Monitoring a high-amplitudeδScuti star for 152 days: discovery of 12 additional modes and modulation effects in the light curve of CoRoT 101155310

The detection of small-amplitude nonradial modes in high-amplitude Delta Sct (HADS) variables has been very elusive until at least five of them were detected in the light curve of V974 Oph obtained from ground-based observations. The combination of radial and nonradial modes has a high asteroseismic potential, thanks to the strong constraints we can put in the modelling. The continuous monitoring of ASAS 192647-0030.0=CoRoT 101155310 (P=0.1258 d, V=13.4) ensured from space by the CoRoT (Convection, Rotation and planetary Transits) mission constitutes a unique opportunity to exploit such potential. The 22270 CoRoT measurements were performed in the chromatic mode. They span 152 d and cover 1208 consecutive cycles. After the correction for one jump and the long-term drift, the level of the noise turned out to be 29 micromag. The phase shifts and amplitude ratios of the coloured CoRoT data, the HARPS spectra, and the period-luminosity relation were used to determine a self-consistent physical model. In turn, it allowed us to model the oscillation spectrum, also giving feedback on the internal structure of the star. In addition to the fundamental radial mode f1=7.949 c/d with harmonics up to 10f1, we detected 12 independent terms. Linear combinations were also found and the light curve was solved by means of 61 frequencies (smallest amplitude 0.10 mmag). The newest result is the detection of a periodic modulation of the f1 mode (triplets at +/-0.193 c/d centred on f1 and 2f1), discussed as a rotational effect or as an extension of the Blazhko effect to HADS stars. The physical model suggests that CoRoT 101155310 is an evolved star, with a slight subsolar metallic abundance, close to the terminal age main sequence. All the 12 additional terms are identified with mixed modes in the predicted overstable region.

Are pre-main-sequence stars older than we thought?

We fit the colour–magnitude diagrams of stars between the zero-age main-sequence and terminal-age main sequence in young clusters and associations. The ages we derive are a factor of 1.5–2 longer than the commonly used ages for these regions, which are derived from the positions of pre-main-sequence stars in colour–magnitude diagrams. From an examination of the uncertainties in the main-sequence and pre-main-sequence models, we conclude that the longer age scale is probably the correct one, which implies that we must revise upwards the commonly used ages for young clusters and associations. Such a revision would explain the discrepancy between the observational lifetimes of protoplanetary discs and theoretical calculations of the time to form planets. It would also explain the absence of clusters with ages between 5 and 30 Myr. We use the τ2 statistic to fit the main-sequence data, but find that we must make significant modifications if we are to fit sequences which have vertical segments in the colour–magnitude diagram. We present this modification along with improvements to the methods of calculating the goodness-of-fit statistic and parameter uncertainties. Software implementing the methods described in this paper is available from http://www.astro.ex.ac.uk/people/timn/tau-squared/.

HD 50844: a new look atδScuti stars from CoRoT space photometry

Context. Aims. This work presents the results obtained by CoRoT on HD 50844, the only δ Sct star observed in the CoRoT initial run (57.6 d). The aim of these CoRoT observations was to investigate and characterize for the first time the pulsational behaviour of a δ Sct star, when observed at a level of precision and with a much better duty cycle than from the ground.Methods. The 140 016 datapoints were analysed using independent approaches (SigSpec software and different iterative sine-wave fittings) and several checks performed (splitting of the timeseries in different subsets, investigation of the residual light curves and spectra). A level of 10-5 mag was reached in the amplitude spectra of the CoRoT timeseries. The space monitoring was complemented by ground-based high-resolution spectroscopy, which allowed the mode identification of 30 terms. Results. The frequency analysis of the CoRoT timeseries revealed hundreds of terms in the frequency range 0–30 d-1 . All the cross-checks confirmed this new result. The initial guess that δ Sct stars have a very rich frequency content is confirmed. The spectroscopic mode identification gives theoretical support since very high-degree modes (up to ) are identified. We also prove that cancellation effects are not sufficient in removing the flux variations associated to these modes at the noise level of the CoRoT measurements. The ground-based observations indicate that HD 50844 is an evolved star that is slightly underabundant in heavy elements, located on the Terminal Age Main Sequence. Probably due to this unfavourable evolutionary status, no clear regular distribution is observed in the frequency set. The predominant term ( d-1 ) has been identified as the fundamental radial mode combining ground-based photometric and spectroscopic data. Conclusions.

Chemical evolution of high-mass stars in close binaries - I. The eclipsing binary V453 Cygni

The eclipsing and double-lined spectroscopic binary system V453 Cygni consists of two early B-type stars, one of which is nearing the terminal age main sequence and one which is roughly halfway through its main-sequence lifetime. Accurate measurements of the masses and radii of the two stars are available, which makes a detailed abundance analysis both more interesting and more precise than for isolated stars. We have reconstructed the spectra of the individual components of V453 Cyg from the observed composite spectra using the technique of spectral disentangling. From these disentangled spectra, we have obtained improved effective temperature measurements of 27 900 ± 400 and 26 200 ± 500 K, for the primary and secondary stars, respectively, by fitting non local thermodynamic equilibrium theoretical line profiles to the hydrogen Balmer lines. Armed with these high-precision effective temperatures and the accurately known surface gravities of the stars we have obtained the abundances of helium and metallic elements. A detailed abundance analysis of the primary star shows a normal (solar) helium abundance if the microturbulence velocity derived from metallic lines is used. The elemental abundances show no indication that CNO-processed material is present in the photosphere of this high-mass terminal age main-sequence star. The elemental abundances of the secondary star were derived by a differential study against a template spectrum of a star with similar characteristics. Both the primary and secondary components display elemental abundances which are in the ranges observed in the Galactic OB stars.

Angular momentum evolution of Algol binaries

We have compiled the well-determined absolute parameters of Algol-type binaries. The lists contain the parameters of 74 detached and 61 semidetached close binaries. The double-lined eclipsing binaries provide not only the most accurate determinations of stellar mass, radius and temperatures but also distance-independent luminosity for each of their individual components. The distributions of the primary and secondary masses of detached binaries (DBs) are similar, whilst the secondary masses of the semidetached binaries (SDBs) are mostly smaller than 2 M ⊙ with a peak in the M 2 -bin (0.21-1.0). The components of the DBs are almost all located ' in the main-sequence band. On the contrary, the secondary components of the SDBs have larger radii and luminosity with respect to the same mass and the same effective temperature of main-sequence counterparts. They occupy a region of the Hertzsprung-Russell diagram between terminal-age main sequence and giants. Moreover, the total angular momenta and specific angular momenta are larger for the SDBs of orbital periods with P > 5 d than those of the shorter period ones. The specific angular momenta of SDBs with periods longer than 5 d are 65 per cent greater than that of the short period group with the same mass. The DBs and the SDBs with orbital periods longer and shorter than 5 d are separated into three groups in the J/M 5/3 - q diagram. The SDBs with mass ratios greater than 0.3 and P > 5 d have almost the same angular momentum to those of DBs. However, the SDBs with short periods have the smallest angular momentum even though they have the same mass ratios. This result reveals that angular momentum loss (AML) considerably affects the evolution of close binary systems. Recently, Chen, Li & Qian suggested that, in addition to magnetic braking, a circumbinary disc may play an important role in AML from Algol-type binaries. Their calculations indicated that the evolution of Algol-type binaries can be significantly affected by the circumbinary disc. Our results show that the evolution of close binaries begins as a DB and losing angular momentum, first via stellar wind and then magnetic braking plus circumbinary disc the period is shortened and orbit shrinks. Thereafter, the evolution of the system is accelerated and mass transfer rates are enhanced which result in a smaller mass ratios.

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based on moderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6-73 Myr. We made fits of the observed lines He I λλ4026, 4387, 4471, and Mg II λ4481, using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 M☉) spin down during their main-sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main-sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin-up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.

The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters

Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. Of these, five clusters are probably not physically associated groups and our results for two other clusters are not reliable, but we identify 52 definite Be stars and an additional 129 Be candidates in the remaining clusters. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation. Our results indicate a possible increase in the fraction of Be stars with increasing cluster distance from the Galactic center (in environments of decreasing metallicity). However, the trend is not significant and could be ruled out due to the intrinsic scatter in our data. We also find no relationship between the fraction of Be stars and cluster density.

On the formation and evolution of magnetic chemically peculiar stars in the solar neighborhood

In order to put strict observational constraints on the evolutionary status of the magnetic chemically peculiar stars (CP2) of the upper main sequence, we have investigated a well established sample of galactic field CP2 objects within a radius of 200 pc from the Sun in the (X, Y )p lane. In total, 182 stars with accurate parallax measurements from the Hipparcos satellite were divided into Si, SiCr and SrCrEu subgroups based on classification resolution data from the literature. Primarily, it was investigated if the CP2 phenomenon occurs at very early stages of the stellar evolution, significantly before these stars reach 30% of their life-time on the main sequence. This result is especially important for theories dealing with stellar dynamos, angular momentum loss during the pre- as well as main sequence and stellar evolutionary codes for CP2 stars. For the calibration of the chosen sample, the well-developed framework of the Geneva 7-color and Stromgren uvbyβ photometric system was used. We are able to show that the CP2 phenomenon occurs continuously at the zero age main sequence for masses between 1.5 and 4.5 M� .T he magnetic field strengths do not vary significantly during the evolution towards the terminal age main sequence. Only the effective temperature and magnetic field strength seem to determine the kind of peculiarity for those stars. We found several effects during the evolution of CP2 stars at the main sequence, i.e. there are two “critical” temperatures where severe changes take place. There is a transition between Si, SiCr and

근접촉쌍성(NCBs)의 물리적 특성에 대한 통계적 분석

66개의 근접촉쌍성(이하 NCBb)의 절대물리량과 궤도요소를 여러 목록으로부터 수집 분석하여 여러 물리량 사이의 상관관계와 NCBs의 물리적 특성을 조사하였다 NCBs의 질량비-광도비 관계는 <TEX>$L_2/L_1{\lapprox}(M_2/M_1)^{1.45}$</TEX>로 나타났으며 CE형에 더 근접한 특성을 보였다. NCBs의 여러 물리량 사이의 상관관계에 따르면 F형이 A형에 비하여 주성과 반성의 질량, 반경, 광도 및 온도의 차이가 작게 나타났다. NCBs의 H-R도에 따르면 A형이 F형에 비하여 다소 더 진화된 종년주계열(TAMS)에 가까이 나타나고 있다. The absolute dimensions and orbital elements of 66 near contact binaries, collected from literatures, have been analyzed to investigate the physical characteristics of the near contact binaries as well as co-relations among physical parameters. The relationship between the mass ratio and luminosity ratio of the near contact binary systems have been obtained as a <TEX>$L_2/L_1{\approx}(M_2/M_1)^{1.45}$</TEX>, which is similar to that of the early type contact binary system. The physical parameters of the new contact binaries show that the difference in mass, radius, luminosity and temperature between the primary and the secondary components for the F type NCBs are smaller than those for the A type NCBs. In H-R diagram, the components of the A types are located closer to the terminal age main sequence than those of the F types.

Preparing theCOROTSpace Mission: New Variable Stars in the Galactic Anticenter Direction

The activities related to the preparation of the asteroseismic, photometric space mission COROT are described. Photoelectric observations, wide-field CCD photometry, uvbyβ calibrations, and further time series have been obtained at different observatories and telescopes. They have been planned to complete the COROT program in the direction of the Galactic anticenter. In addition to suitable asteroseismic targets covering the different evolutionary stages between zero-age main sequence and terminal-age main sequence, we discovered several other variable stars, both pulsating and geometric. We compared results on the incidence of variability in the Galactic center and anticenter directions. Physical parameters have been obtained, and evolutionary tracks fitting them have been calculated. The peculiarities of some individual stars are pointed out.

On the detection of pure sine waves embedded in a spectrum of stochastically excited p modes

The mode identification and fitting of solar oscillations provide an observational tool to derive the physical properties of the Sun’s interior. What has been devised for helioseismology can now be used for future asteroseismology ground-based instruments and space missions, such as the High-Accuracy Radial Velocity Planetary Searcher (HARPS) and Convection, Rotation and Planetary Transits (COROT). In the coming decade, numerous stars will be observed for which new tools will be needed. For instance, it is very likely that g and p modes will be detected, rendering the identification difficult. In addition, modes having both characteristics, known as mixed modes, are also likely to be detected in evolved stars; these latter modes are crucial for the understanding of the internal structure of the stars as they propagate deeply inside the stars, unlike the p modes. Unfortunately, the mixing will not only occur physically but also in the frequency domain: mixed modes will appear very close to stochastically excited p modes. In this paper, we have devised a new technique for detecting long-lived modes (mixed or g modes) embedded in a common p-mode spectrum. The technique has been validated using Monte Carlo simulations. In the framework of the hare-and-hound exercise of COROT, this technique has been applied to synthetic time series of the evolved solar-like star HD 57006. The results show that we can detect most of the mixed modes embedded by the hare. We also discuss the impact of how the long-lived modes are excited on the detection level. The technique described here can be applied to stars with a mass greater than 1.2 M� ,h a ving evolved possibly beyond the terminal-age main sequence, such as Procyon. Ke yw ords: methods: statistical ‐ stars: oscillations.

An analysis of the light curves of the overcontact binary system V2388 Ophiuchi

We present four seasons' (2000-2003) ground-based photometry of the short-period contact binary V2388 Oph. The system is the brighter component of visual binary HIP 87655. The magnitude difference between the visual companion and the eclipsing pair was estimated to be 1. m 19, 1. m 09 and 1. 09 in B, V and R bandpasses, respectively. The light curves in BVR are solved by the WD code. Combining the parameters found by the light curve analysis with those of the radial velocity solution we derived the masses and radii of the star components as: M1 = 1.80(2) M� , M2 = 0.34(1) M� , R1 = 2.60(2) R� , R2 = 1.30(1) R� . The primary component appears to be more evolved in the mass-radius, mass-luminosity planes and also in the HR diagram. It lies near the terminal-age main-sequence, which agrees well with the position of deeper-contact A-type systems. The large fill-out factor of 0.65 does also support this classification.

CCD photometric search for peculiar stars in open clusters

The time scale on which chemical peculiarity in upper main sequence stars begins is unknown. Results from the Hipparcos measurements indicate that the classical chemically peculiar (CP) stars occupy the whole area from the zero age main sequence to the terminal age main sequence (Gomez et al. [CITE]). However, Hubrig et al. ([CITE]) reported that the magnetic fields appear only in stars that have already completed at least approximately 30% of their main sequence life time. Previous observations in the very young Orion OB1 and Scorpio-Centaurus associations (Joncas & Borra 1981; Borra et al. 1982) reveal a percentage of chemically peculiar stars that is significantly smaller (less than 5%) than that of the galactic field (up to 25%). This already indicates that the phenomenon needs at least several 10 6 yr to appear. Since no further observations have been published since then, we have observed the five very young (1 Myr ≤ age ≤ 25 Myr) open clusters Collinder 272, Pismis 20, Lynga 14, NGC 6396 and NGC 6611. Photometry from 250 CCD frames in the three filter $\Delta a$-system results in very low detection limits (0.008 to 0.015 mag). We find no single photometrically chemically peculiar candidate in approximately 460 observed cluster members in the five program clusters. From the number of observed objects we deduce upper limits between 0.6% and 2.4% for the incidence of CP stars for the four observed aggregates.

First orbital elements for theλ Bootis spectroscopic binary systems HD 84948 and HD 171948

We present high-resolution spectroscopic observations of the two Bootis type spectroscopic binary systems HD 84948 and HD 171948. Both systems consist of two true Bootis stars, as has already been proven by a detailed abundance analysis taking into account the binary nature. Nevertheless, we have obtained non- LTE abundances for carbon and oxygen in order to investigate those important elements. The abundances t excellently into the typical pattern for this group, leaving no doubt that all four components are true Bootis stars. With the help of the derived orbital elements it is possible to establish the ages of the two systems. For the rst time we can estimate the evolutionary status of Galactic eld Bootis stars. The origin of those nonmagnetic, metal-weak, Population I, late B- to early F-type stars is still controversial. The two widely discussed theories explaining the Bootis phenomenon (diusion together with either accretion of circumstellar matter or mass- loss) predict signicantly dierent evolutionary stages for this group of objects (close to either the Zero-Age Main Sequence or Terminal-Age Main Sequence). It is already known that very young members of the group exist in the Orion OBI association and probably in NGC 2264. Hipparcos data established six further Galactic eld Bootis stars which are close to the Zero-Age Main Sequence, whereas the evolutionary status for the other objects remained undetermined. The Hipparcos data reveal that the Galactic space motions of both the systems that we discuss here are typical of those of Population I objects. The photometric data from the Hipparcos mission conrm the pulsation previously discovered for HD 84948 with a period of about 110 min and a V -amplitude of about 14 mmag. For HD 84948, we estimate from the mass ratio an age of about 1 Gyr, ruling out a possible Pre-Main-Sequence status. HD 171948 has an age of about 0.01 to 0.1 Gyr which is close to the Zero-Age Main Sequence. We therefore conclude that the Bootis phenomenon can be found continuously from very early stages to the Terminal-Age Main Sequence, suggesting that dierent mechanisms might work at dierent stages of stellar evolution producing the same abundance pattern.

A spectroscopic survey for λ Bootis stars

In the third paper of a series dedicated to the spectroscopic survey for new Bootis stars, we present all new and conrmed members of the group as well as a detailed analysis of the observed sample. The nature of this small group of chemically peculiar stars of the upper main sequence still challenges our understanding of processes like diusion, mass-loss and accretion. The typical abundances pattern (nearly solar values for C, N, O and S whereas the Fe-peak elements are moderate to strong underabundant) can still not be explained by any proposed theory. Hence, the signicant increase of new members gives the opportunity to investigate the group properties in more detail. We report the discovery of 26 new members of the group and the conrmation of 18 candidates from the literature. This almost triples the number of known Bootis stars. The existence of one member in the young open cluster NGC 2264 and four members in the Orion OB1 association proves that the Bootis phenomenon already works at very early stages of stellar evolution. Recent results from the Hipparcos mission have shown that the well established Bootis stars of the Galactic eld comprise the whole area from the Zero Age Main Sequence to the Terminal Age Main Sequence ( 10 9 yr for an A-type star). There is a continuous transition between very young and rather evolved evolutionary stages. We nd that the overall percentage of Bootis type among all normal type stars in the spectral range from B8 to F4 is 2% in the Galactic eld as well as in open clusters. Furthermore, 44 metal-weak objects are listed which might be connected with the Bootis phenomenon. Our biased sample (chosen by photometric boxes) is not distinguished from all A-type stars in the corresponding spectral region by the rotational velocity distribution. Only for the luminosity classes IV and III (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature.