Λ Bootis Stars Research Articles

Preliminary Statistical Analysis of Lambda Bootis Stars

To enlarge our database of Chemically Peculiar stars, we compiled published data concerning the Lambda Bootis stars observed by high-resolution spectroscopy techniques during last decades. In total, 62 Lambda Bootis stars have been added to the database. To deepen our knowledge on statistical overview of the abundance anomalies versus the physical parameters of stars, our data were compared with previous compilations, as well as with Gaia DR2 data. Different statistical tests were applied on our data for 20 chemical elements for the search of interesting correlations for different physical parameters in the atmospheres of those stars. We confirmed all previous results and obtained correlations between different physical parameters, but because of the lack of the data, we could not find new correlations. We hope, that Gaia EDR3 data, on which we are working now, will help us to improve our database and to understand the nature of all Chemically Peculiar stars.

λ Bootis stars in the SuperWASP survey

We have analysed around 170 000 individual photometric WASP measurements of 15 well established lambda Bootis stars to search for variability. The lambda Bootis stars are a small group of late-B to early-F, Pop. I, stars that show moderate to extreme (surface) underabundances (up to a factor 100) of most Fe-peak elements, but solar abundances of lighter elements (C, N, O and S). They are excellent laboratories for the study of fundamental astrophysical processes such as diffusion, meridional circulation, stellar winds, and accretion in the presence of pulsation. From the 15 targets, eight are variable and seven are apparently constant with upper limits between 0.8 and 3.0 mmag. We present a detailed time series analysis and a comparison with previously published results. From an asteroseismologic study we conclude that the found chemical peculiarities are most probably restricted to the surface.

SEARCHING FOR DUST AROUND HYPER METAL POOR STARS

We examine the mid-infrared fluxes and spectral energy distributions for stars with iron abundances [Fe/H] <−5, and other metal-poor stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excesses. These non-detections rule out many types of circumstellar disks, e.g., a warm debris disk (T ⩽ 290 K), or debris disks with inner radii ⩽1 AU, such as those associated with the chemically peculiar post-asymptotic giant branch spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g., a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 μm is detected at the 2σ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.

Investigating the possible connection betweenλBootis stars and intermediate Population II type stars

The lambda Bootis (LB) stars are located at the upper main sequence of the H-R diagram and exhibit a peculiar abundance pattern. The light elements (C, N, O, and S) present solar abundances whereas all other elements are moderately to strongly underabundant. It has not yet been determined whether that abundance pattern is intrinsic, or is restricted to the stellar surface. If we follow the hypothesis that the LB stars are intrinsically metal-weak, then there should be a connection with the intermediate Population II and F-weak objects. Such a possible affinity has not been previously investigated. We present detailed elemental abundances, including those of the light elements carbon and oxygen, for 38 bright intermediate Population II and F-weak objects. In addition, we investigate the kinematic characteristics of the groups. From photometric, spectroscopic, and kinematic data, there is no distinction between the intermediate Population II and F-weak type stars. We therefore conclude that the two groups are identical. However, it is possible to distinguish the LB stars from the intermediate Population II stars on the basis of elemental abundances, though not in terms of their kinematics. The LB stars seem to be distinct from the intermediate Population II group. Further asteroseismologic investigations and analyses of spectroscopic binary systems are needed to strengthen this conclusion.

HD 54272, a classical λ Bootis star and γ Doradus pulsator

We detect the second known lambda Bootis star (HD 54272) which exhibits gamma Doradus-type pulsations. The star was formerly misidentified as a RR Lyrae variable. The lambda Bootis stars are a small group (only 2 per cent) of late B to early F-type, Population I stars which show moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The photometric data from the Wide Angle Search for Planets (WASP) and All Sky Automated Survey (ASAS) projects were analysed. They have an overlapping time base of 1566 d and 2545 d, respectively. Six statistically significant peaks were identified (f1 = 1.410 116 c/d, f2 = 1.283 986 c/d, f3 = 1.293 210 c/d, f4 = 1.536 662 c/d, f5 = 1.157 22 c/d and f6 = 0.226 57 c/d). The spacing between f1 and f2, f1 and f4, f5 and f2 is almost identical. Since the daily aliasing is very strong, the interpretation of frequency spectra is somewhat ambiguous. From spectroscopic data, we deduce a high rotational velocity (250+-25 km/s) and a metal deficiency of about -0.8 to -1.1 dex compared to the Sun. A comparison with the similar star, HR 8799, results in analogous pulsational characteristics but widely different astrophysical parameters. Since both are lambda Bootis-type stars, the main mechanism of this phenomenon, selective accretion, may severely influence gamma Doradus-type pulsations.

THE CHARA ARRAY ANGULAR DIAMETER OF HR 8799 FAVORS PLANETARY MASSES FOR ITS IMAGED COMPANIONS

HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799's limb-darkened angular diameter to be 0.342 +/- 0.008 mas; this is the smallest interferometrically measured stellar diameter to date, with an error of only 2%. By combining our measurement with the star's parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (7193 +/- 87 K, consistent with F0), luminosity (5.05 +/- 0.29 L_Sun), and the extent of the habitable zone (1.62 AU to 3.32 AU). These improved stellar properties permit much more precise comparisons with stellar evolutionary models, from which a mass and age can be determined, once the metallicity of the star is known. Considering the observational properties of other lambda Bootis stars and the indirect evidence for youth of HR 8799, we argue that the internal abundance, and what we refer to as the effective abundance, is most likely near-solar. Finally, using the Yonsei-Yale evolutionary models with uniformly scaled solar-like abundances, we estimate HR 8799's mass and age considering two possibilities: 1.516 +0.038/-0.024 M_Sun and 33 +7/-13 Myr if the star is contracting toward the zero age main-sequence or 1.513 +0.023/-0.024 M_Sun and 90 +381/-50 Myr if it is expanding from it. This improved estimate of HR 8799's age with realistic uncertainties provides the best constraints to date on the masses of its orbiting companions, and strongly suggests they are indeed planets. They nevertheless all appear to orbit well outside the habitable zone of this young star.

HD 210111: a new λ Bootis-type spectroscopic binary system

The small group of lambda Bootis stars comprises late B to early F-type stars, with moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The main mechanisms responsible for this phenomenon are atmospheric diffusion, meridional mixing and accretion of material from their surroundings. Especially spectroscopic binary (SB) systems with lambda Bootis type components are very important to investigate the evolutionary status and accretion process in more details. For HD 210111, also delta Scuti type pulsation was found which gives the opportunity to use the tools of asteroseismology for further investigations. The latter could result in strict constraints for the amount of diffusion for this star. Together with models for the accretion and its source this provides a unique opportunity to shed more light on these important processes. We present classification and high resolution spectra for HD 210111. A detailed investigation of the most likely combinations of single star components was performed. For this, composite spectra with different stellar astrophysical parameters were calculated and compared to the observations to find the best fitting combination. HD 210111 comprises two equal (within the estimated errors) stars with T(eff)=7400K, logg=3.8dex, [M/H]=-1.0dex and vsini=30km/s. This result is in line with other strict observational facts published so far for this object. It is only the third detailed investigated lambda Bootis type SB system, but the first one with a known IR-excess.

INFRARED EMISSION BY DUST AROUND λ BOOTIS STARS: DEBRIS DISKS OR THERMALLY EMITTING NEBULAE?

We present a model that describes stellar infrared excesses due to heating of the interstellar (IS) dust by a hot star passing through a diffuse IS cloud. This model is applied to six lambda Bootis stars with infrared excesses. Plausible values for the IS medium (ISM) density and relative velocity between the cloud and the star yield fits to the excess emission. This result is consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A- to F-type stars with large underabundances of Fe-peak elements) owe their characteristics to interactions with the ISM. This proposal invokes radiation pressure from the star to repel the IS dust and excavate a paraboloidal dust cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar photosphere. However, the measurements of the infrared excesses can also be fit by planetary debris disk models. A more detailed consideration of the conditions to produce lambda Bootis characteristics indicates that the majority of infrared-excess stars within the Local Bubble probably have debris disks. Nevertheless, more distant stars may often have excesses due to heating of interstellar material such as in our model.

Could the Ultra–Metal‐Poor Stars be Chemically Peculiar and Not Related to the First Stars?

Chemically peculiar stars define a class of stars that show unusual elemental abundances due to stellar photospheric effects and not due to natal variations. In this paper, we compare the elemental abundance patterns of the ultra-metal-poor stars with metallicities -->[ Fe/H ] ~ ? 5 to those of a subclass of chemically peculiar stars. These include post-AGB stars, RV Tauri variable stars, and the Lambda Bootis stars, which range in mass, age, binarity, and evolutionary status, yet can have iron abundance determinations as low as -->[ Fe/H ] ~ ? 5. These chemical peculiarities are interpreted as due to the separation of gas and dust beyond the stellar surface, followed by the accretion of dust-depleted gas. Contrary to this, the elemental abundances in the ultra-metal-poor stars are thought to represent yields of the most metal-poor supernovae and, therefore, observationally constrain the earliest stages of chemical evolution in the universe. Detailed chemical abundances are now available for HE 1327?2326 and HE 0107?5240, the two extreme ultra-metal-poor stars in our Galaxy, and for HE 0557?4840, another ultra-metal-poor star found by the Hamburg/ESO survey. There are interesting similarities in their abundance ratios to those of the chemically peculiar stars; e.g., the abundances of the elements in their photospheres are related to the condensation temperature of that element. If these three stars are chemically peculiar, then their CNO abundances suggest true metallicities of -->[ X/H ] ~ ? 2 to ?4. It is important to establish the nature of these stars, since they are used as tests of the early chemical evolution of the Galaxy.

Pulsation of the Lambda Bootis star HD 210111

CONTEXT. The Lambda Bootis stars are a small spectroscopic subgroup of Population I A-type stars and show significant underabundances of metals. Many are Delta Scuti pulsators. AIMS. HD 210111 was selected for a detailed multisite pulsation study to determine whether its pulsation properties differ from those of normal A stars. METHODS. 262 hours of high-precision photometry were obtained at the SAAO and SSO observatories. RESULTS. 13 statistically significant pulsation frequencies were detected with very small photometric amplitudes from 1 to 7 millimag in the visual. A comparison with earlier 1994 measurements indicates a small increase in amplitude. As a byproduct, one of the comparison stars, HD 210571, was discovered to be a millimag variable with a frequency of 1.235 c/d and is probably a new Gamma Doradus variable. The observed wide range of excited frequencies from 12 to 30 c/d in HD 210111 can be explained with both the single- and double-star hypothesis. HD 210111 is in a similar evolutionary status to FG Vir, which also shows a wide range of excited frequencies with a similar frequency spacing near 4 c/d. This is interpreted as successive radial orders of the excited nonradial modes. In the double-star hypothesis previously evoked for HD 210111, the low and the high frequencies originate in different stars: here HD 210111 would resemble Theta^2 Tau. CONCLUSIONS. The pulsation of the Lambda Bootis star HD 210111 does not differ from that of normal Delta Scuti stars.

On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics ofλ Bootis type stars

The group of lambda Bootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. (1998, AA we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen, 2001, A&A, 373, 633) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the lambda Bootis stars with those of a sample of delta Scuti type objects. We find that at least 70% of all lambda Bootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q < 0.020d). Only a few stars, if any, pulsate in the fundamental mode. Compared to the delta Scuti stars, the cool and hot borders of the instability strip of the lambda Bootis stars are shifted by about 25mmag, towards smaller (b-y)0. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Stroemgren uvbybeta systems, we have derived [Z] values which describe the surface abundance of the heavier elements for the group members. We find that the Period-Luminosity-Colour relation for the group of lambda Bootis stars is within the errors identical with that of the normal delta Scuti stars. No clear evidence for a statistically significant metallicity term was detected.

The accretion/diffusion theory forλ Bootis stars in the light of spectroscopic data

Most of the current theories suggest the lambda Bootis phenomenon to originate from an interaction between the stellar surface and its local environment. In this paper, we compare the abundance pattern of the lambda Bootis stars to that of the interstellar medium and find larger deficiencies for Mg, Si, Mn and Zn than in the interstellar medium. A comparison with metal poor post-AGB stars showing evidence for circumstellar material indicates a similar physical process possibly being at work for some of the lambda Bootis stars, but not for all of them. Despite the fact that the number of spectroscopically analysed lambda Bootis stars has considerably increased in the past, a test of predicted effects with observations shows current abundance and temperature data to be still controversial.

New λ Bootis stars with a shell

We publish here the second part of our spectroscopic survey at high dispersion of some known and suspected lambda Bootis stars with a view to detecting circumstellar shell features. Eight stars of our sample exhibit such features. These stars are fast rotators, a result which is in line with Holweger and Rentzsch-Holm's study (1995). The analysis of the photometric data has allowed us to confirm the exclusion of a few stars misclassified from the lambda Bootis group.

A consolidated catalogue of lambda Bootis stars

λ Bootis stars challenge our understanding of diffusion and accretion processes related to stars and their circumstellar environment, and they are interesting components of the classical instability strip. Attempts to derive group properties with statistical methods are severely limited by the small number of unambiguously identified λ Bootis stars. In general, the subject appears to be obscured by incorrect memberships and it is therefore essential to provide a sufficiently large catalogue of definitive group members before modeling the λ Bootis phenomenon. This paper describes the first steps towards this goal, based on our current knowledge of well investigated members, leading to a concise definition of λ Bootis stars: Pop I hydrogen burning metal poor (except of C,N,O and S) A-type stars. The definition does not depend on phenomenological features, like flux depressions, colour excesses, values, etc. Based on a new homogeneous catalogue with 45 λ Bootis stars, we discuss classification criteria which can be used for a spectroscopic and photometric all-sky survey for λ Bootis stars in the field and in clusters of different ages.

A search for Lambda Bootis stars in OB associations

view Abstract Citations (61) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for Lambda Bootis Stars in OB Associations Gray, R. O. ; Corbally, C. J. Abstract A search for Lambda Bootis stars in two OB associations, Orion OB1 and Lacerta OB1, has been carried out. One Lambda Bootis star has been discovered in the Orion OB1 association. In addition, nine A-type stars which appear to be slightly metal weak have been found in the two associations. The Lambda Bootis star is situated on the Zero-Age Main Sequence (ZAMS). The discovery of these stars strengthens the hypothesis that the Lambda Bootis stars are related to a short-lived evolutionary phase associated with the arrival of the star on the ZAMS. Publication: The Astronomical Journal Pub Date: August 1993 DOI: 10.1086/116668 Bibcode: 1993AJ....106..632G Keywords: B Stars; Main Sequence Stars; O Stars; Stellar Evolution; Stellar Spectra; Charge Coupled Devices; Metallicity; Astrophysics; STARS: CHEMICALLY PECULIAR; STARS: ATMOSPHERES full text sources ADS | data products SIMBAD (68)

Particle transport and the Lambda Bootis phenomenon. I - The diffusion/mass-loss model revisited

view Abstract Citations (43) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Particle Transport and the lambda Bootis Phenomenon. I. The Diffusion/Mass-Loss Model Revisited Charbonneau, P. Abstract In this paper, results of two-dimensional linear time-dependent particle transport calculations in a Lambda Bootis star are presented for two representative elements, titanium and calcium. These demonstrate unambiguously that the inclusion of meridional circulation in the original diffusion/mass-loss model of Lambda Bootis stars has profound consequences on the abundance evolution. More specifically, circulation prevents the appearance, at any epoch of main-sequence evolution, of the underabundance patterns characteristic of Lambda Bootis stars. This indicates that the diffusion/mass-loss model for these objects must be either abandoned or significantly modified. Caveats and possible alternatives are discussed, and a few observational tests are suggested. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172399 Bibcode: 1993ApJ...405..720C Keywords: Abundance; Astronomical Models; Meridional Flow; Peculiar Stars; Stellar Rotation; Transport Properties; Calcium; Diffusion; Mass Transfer; Stellar Evolution; Titanium; Turbulent Mixing; Astrophysics; DIFFUSION; STARS: ABUNDANCES; STARS: INTERIORS; STARS: MASS LOSS; STARS: PECULIAR; STARS: ROTATION full text sources ADS | data products SIMBAD (1) Related Materials (1) Part 2: 1993ApJ...413..376T

Carbon and Silicon in Normal a Stars and in Lambda Bootis Stars

AbstractThe anticorrelation of C and Si previously inferred from high-resolution spectroscopy of sharp-lined A stars classified as normal or mild Am is also found among the metal-deficient, more rapidly rotating λ Boo stars. The entire sample of program stars covers the range [C/Si] = − 1.14 (Sirius, mild Am star) to + 1.35 (HR 7400, λ Boo-type). Iron varies parallel with silicon. With respect to surface chemistry the λ Boo stars do not form a distinct group but rather represent the lower end in a continuous sequence of ‘metal’ abundance extending over more than 1.5 dex.We argue that differential accretion of gas and dust in late stages of star formation or during the main-sequence phase is responsible for most of the abundance variations of the lighter elements up to the iron group. Enhanced or impeded accretion of dust leads to a metal-rich or metal-poor surface composition, respectively. The heavy trace elements Sr and Ba show additional anomalies indicating that other processes like gravitational and radiative separation are also involved.

Chemical Separation vs Rotation in a and F-Stars

AbstractThe role played by rotationally-induced mixing in the diffusion-based models for non-magnetic chemically peculiar stars is investigated. This paper focuses on one specific rotationally controlled mixing mechanism, namely thermally-driven meridional circulation. Its effects on the time evolution of chemical abundances are illustrated by means of three specific examples. The first two concern the diffusion model for FmAm stars, where it is shown that while circulation has a determining influence on the settling of Helium, it has no significant effect on the diffusion of heavier metals once the He superficial convection zone has disappeared. The third example is concerned with the diffusion/mass loss model for λBootis stars. It is shown that the inclusion of circulation prevents the appearance of generalized underabundances at any epoch of the evolution, indicating that the diffusion/mass loss model for these objects must be abandoned.

A simple accretion/diffusion model for Lambda Bootis stars

It has recently been suggested that the peculiar abundance patterns observed in the Lambda Bootis stars could be understood in terms of accretion of gas previously depleted in metals by means of grain formation in the interstellar medium. A simple analytical model is presented, describing the evolution of elemental abundances in these stars, under the combined influence of accretion and chemical separation. The only arbitrary parameter involved is the accretion rate. A rate of order 10 to the {minus}13th solar mass/yr is found to naturally reproduce many peculiar characteristics of Lambda Bootis stars, in particular their restriction to the spectral type range A0{minus}F0. This lends quantitative support to the accretion hypothesis as a key component toward an understanding of the Lambda Bootis phenomenon. 18 refs.

The chemical composition of three Lambda Bootis stars

view Abstract Citations (259) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Chemical Composition of Three Lambda Bootis Stars Venn, Kim A. ; Lambert, David L. Abstract Abundance analyses are reported for three certain members (Lambda Boo, 29 Cyg, Pi1 Ori) of the class of rapidly rotating, metal-poor A-type stars known as Lambda Bootis stars. Model atmosphere analysis of high-resolution, high signal-to-noise spectra shows that the metal deficiencies are more severe than previously reported: Fe/H = -2.0, -1.8, -1.3 for Lambda Boo, 29 Cyg, and Pi1 Ori, respectively. Other metals (Mg, Ca, Ti, and Sr) are similarly underabundant, with Na often having a smaller underabundance. C, N, O, and S have near-solar abundances. Vega is shown to be a mild Lambda Boo star. The abundance anomalies of the Lambda Boo stars resemble those found for the interstellar gas in which the metals are depleted through formation of interstellar grains. It is suggested that the Lambda Boo stars are created when circumstellar (or interstellar) gas is separated from the grains and accreted by the star. The bulk of the interstellar grains comprises a circumstellar cloud or disk that is detectable by its infrared radiation. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169334 Bibcode: 1990ApJ...363..234V Keywords: A Stars; Main Sequence Stars; Stellar Composition; Chemical Composition; Infrared Astronomy; Interstellar Gas; Metallicity; Stellar Mass Accretion; Stellar Rotation; Astrophysics; STARS: ABUNDANCES; STARS: ACCRETION; STARS: PECULIAR A full text sources ADS | data products SIMBAD (4)