Bootis Phenomenon Research Articles

HD 210111 compared to FG Vir and θ2 Tau: a test to the λ Bootis phenomenon

The λ Bootis stars are a spectroscopic subgroup of chemically peculiar Population I A-type stars. They show significant underabundances of Fe-peak elements and solar abundances of lighter elements. Theoretical explanations include surface phenomena and binarity. We compared the pulsating λ Bootis star HD 210111 with two normal δ Scuti pulsators. This comparison shows that the pulsational behaviour of HD 210111 is compatible with the λ Bootis phenomenon being a surface effect only.

Modeling the Infrared Bow Shock at δ Velorum: Implications for Studies of Debris Disks and λ Bootis Stars

We have discovered a bow shock shaped mid-infrared excess region in front of δ Velorum using 24 μm observations obtained with the Multiband Imaging Photometer for Spitzer (MIPS). Although the bow shock morphology was only detected in the 24 μm observations, its excess was also resolved at 70 μm. We show that the stellar heating of an ambient interstellar medium (ISM) cloud can produce the measured flux. Since δ Velorum was classified as a debris disk star previously, our discovery may call into question the same classification of other stars. We model the interaction of the star and ISM, producing images that show the same geometry and surface brightness as is observed. The modeled ISM is ~15 times overdense relative to the average Local Bubble value, which is surprising considering the close proximity (24 pc) of δ Velorum. The abundance anomalies of λ Bootis stars have been previously explained as arising from the same type of interaction of stars with the ISM. Low-resolution optical spectra of δ Velorum show that it does not belong to this stellar class. The star therefore is an interesting testbed for the ISM accretion theory of the λ Bootis phenomenon.

Chemical analysis of 24 dusty (pre-)main-sequence stars

We have analysed the chemical photospheric composition of 24 Herbig Ae/Be and Vega-type stars in search for the λ Bootis phenomenon. We present the results of the elemental abundances of the sample stars. Some of the stars were never before studied spectroscopically at optical wavelengths. We have determined the projected rotational velocities of our sample stars. Furthermore, we discuss stars that depict a (selective) depletion pattern in detail. HD 4881 and HD 139614 seem to display an overall deficiency. AB Aur and possibly HD 126367 have subsolar values for the iron abundance, but are almost solar in silicon. HD 100546 is the only clear λ Bootis star in our sample.

The $\lambda$ Bootis stars

A comprehensive overview of the Bootis phenomenon are covered. New observations including detailed elemental (surface) abundances, tests for pulsational instability, fluxes in the IR region, and measurements from the Hipparcos satellite permit one to make a statistical sound analysis of the group properties. Details of the evolutionary status, abundance pattern and tests for developed theories are presented.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The λ Bootis phenomenon: interaction between a star and a diffuse interstellar cloud

The λ Bootis stars, a group of late B- to early F-type Population I stars, have surface abundances that resemble the general metal depletion pattern found in the interstellar medium. Inspired by the recent result that the fundamental parameters of these peculiar stars differ in no respect from those of a comparison sample of normal stars, the hypothesis of an interaction between a star and a diffuse interstellar cloud is considered as a possible explanation of the peculiar abundance pattern. It is found that such a scenario is able to explain the selective accretion of interstellar gas depleted in condensable elements as well as the spectral range of the λ Bootis phenomenon.

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.

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.

Physical parameters of $\mathsf{\lambda}$ Bootis stars

This is the rst of two papers whose main goal is to update and improve the information available on the physical properties of the Bootis stars. The determination of the stellar parameters is of fundamental importance to shed light into the dierent theories proposed to explain the Bootis phenomenon. With this aim, projected rotational velocities, eective temperatures, surface gravities and chemical abundances of a sample of suspected Bootis stars have been calculated. Five objects showing composite spectra typical of binary systems were found in our analysis. The abundance distribution of the program stars does not resemble the chemical composition of the class prototype, Boo, which poses some concerns regarding the idea of a well-dened, chemically homogeneous group of stars. A possible relation between rotational velocities and the Bootis phenomenon has been found. This result would be in agreement with the accretion scenario proposed by Turcotte & Charbonneau (1993).

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.

HR 8799: A Link between γ Doradus Variables and λ Bootis Stars

On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered γ Doradus variable star class, is a λ Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields Teff = 7430 K, log g = 4.35, microturbulent velocity ξt = 2.7 km s-1, and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a Vega-like star in that it shows excess flux at 60 μm, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the λ Bootis phenomenon and the γ Doradus pulsation phenomenon.

Nonvariability among λ Bootis stars

With asteroseismic techniques it is possible to investigate the interior and the evolutionary status of stars via their frequency spectrum. Both information would be very much needed for Bootis stars, a group of metal- poor Population I, A-type stars, since no conclusive the- ory exists explaining the observed abundance anomalies. Geneva and Stromgren photometry place these stars in- side the classical instability strip or at least very close to it. We therefore have started an extensive photometric survey for pulsation in Bootis stars and have discovered so far 13 new variables. In this paper we present results for stars which presum- ably are constant, because we are able to establish only an upper level for possible variability. A typical noise level of 3 mmag for Stromgren b was achieved in the relevant fre- quency domain up to 100 d 1 . Considering the given noise level of our survey, we con- clude that at least 50% of all investigated Bootis stars inside the instability strip are pulsating, making this group remarkable compared to stars with similar spectral types. This may suggest that a low (surface) metallicity has an influence on the pulsation behavior of stars. New models, as well as further observations are needed to clarify the Bootis phenomenon using asteroseismic techniques.

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.

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

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.