A NEAR-INFRARED SPECTROSCOPIC SURVEY OF CLASS I PROTOSTARS

We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ~2000. Our observations span the K-band, and include the Br gamma transition of Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several targets we also present data from Keck/NIRSPEC that provide higher spectral resolution, but a seeing-limited spatial resolution, of the same spectral features. We analyze the Br gamma emission in the context of both disk and infall/outflow models, and conclude that the Br gamma emission traces gas at very small stellocentric radii, consistent with the magnetospheric scale. However some Br gamma-emitting gas also seems to be located at radii of >0.1 AU, perhaps tracing the inner regions of magnetically launched outflows. CO emission is detected from several objects, and we generate disk models that reproduce both the KI and NIRSPEC data well. We infer the CO spatial distribution to be coincident with the distribution of continuum emission in most cases. Furthermore the Br gamma emission in these objects is roughly coincident with both the CO and continuum emission. We present potential explanations for the spatial coincidence of continuum, Br gamma, and CO overtone emission, and explore the implications for the low occurrence rate of CO overtone emission in young stars. Finally, we provide additional discussion of V1685 Cyg, which is unusual among our sample in showing large differences in emitting region size and spatial position as a function of wavelength.

Effects of Mass Loss on Stellar Evolution

Equivalent widths of 16 lines of CI, CII, CIII, CIV, SiII, SiIII, SiIV, AlII, AlIII, FeII, and FeIII, plus centroid and edge velocities of the SiIV and CIV lines, were measured in IUE spectra of 39 Ble–B8e and 18 B1–B8 standard stars. These suggest the following: 1.Certain line ratios of Sill/III, CII/III, A1II/III, and Fell/Ill are very sensitive to spectral type and represent excellent UV criteria for spectral classification.2.UV line strengths and line ratios show that there are no significant differences between the photospheric line spectra of Be and normal, nonemission stars of corresponding type.3.Despite the fact that the SilV and CIV wind lines are variable in the Be stars, certain conclusions can be drawn from a statistical “snapshot” study such as this:a)The SilV and CIV wind lines in the Be stars are correlated with both spectral type and luminosity class in the sense that the hottest stars have the strongest lines, and the giants and subgiants (at least for the B1-B3 stars) have stronger lines than the main-sequence stars, (see Fig. 1 and 2).b)The SilV wind lines persist to spectral type B8 in both the Be stars and the standard stars but are stronger in the Be stars than in the standards for the earlier types (B1-B3). (See Fig. 1)c)The CIV wind lines persist to spectral type B8 in the Be stars but only to B3 in the standard stars. They are stronger in the Be stars than in the standards at all spectral types, (see Fig. 2).d)The equivalent widths of the SilV and CIV wind lines are only very weakly correlated with vsini, if at all, but a threshold in vsini near 150 km s− 1 (as found earlier by Grady et al.) exists below which no large equivalent widths of SilV or CIV may be seen. Assuming that the Be stars are all rapid rotators, such a correlation is essentially a correlation with i and suggests that the winds from Be stars arise preferentially from the equatorial regions. The aforementioned conclusion is supported by a plot of SilV 1394 centroid velocities versus vsini, which shows that stars with large velocity shifts also have large vsini, while those with unshifted lines all have vsini less than about 150 km s− 1. Again, it seems that strong stellar winds are more likely to arise from equatorial than polar regions.e)Additional evidence that the winds are stronger for Be stars of early type comes from the SilV and CIV centroid velocities, which are much larger for the hotter Be stars. The stars with the strongest lines also tend to have the largest velocity shifts, suggesting that winds with more mass are also faster moving.f)The SilV and CIV lines in the standard stars, while they may be asymmetric, never show displaced line centers.g)The edge velocities for both the SilV 1394 and the CIV 1548 Unes increase with earlier spectral type in the Be stars (therefore, stronger winds in the hotter stars), and are considerably higher in the Be stars than in the standard stars, suggesting again that the winds are stronger in the Be stars.h)Shell stars have weaker CIV absorption and smaller edge and centroid velocities than other Be stars, suggesting that they have weaker winds. Since there is considerable evidence that these are stars with cool, low-velocity disks which axe being viewed edge-on or nearly edge-on, the winds may be inhibited and modified by the denser material in the equatorial regions.4.Mg II emission is detected in about half of the program Be stars with long wavelength IUE spectra, and seems not to be correlated with spectral type, vsini, or strength of the SilV wind lines. Since the Mgll emission presumably originates in the cool, low velocity envelope and since Mgll emission also correlates with hydrogen Balmer emission in the Be stars, this suggests that there is no strong physical relationship between the stellar winds and the cool disk.

Equivalent widths of 16 lines of CI, CII, CIII, CIV, SiII, SiIII, SiIV, AlII, AlIII, FeII, and FeIII, plus centroid and edge velocities of the SiIV and CIV lines, were measured in IUE spectra of 39 B1e—B8e and 18 Bl—B8 standard stars. These suggest the following: 1. Certain line ratios of Sill/III, CII/III, AlII/III, and FeII/III are very sensitive to spectral type and represent excellent UV criteria for spectral classification. 2. UV line strengths and line ratios show that there are no significant differences between the photospheric line spectra of Be and normal, non-emission stars of corresponding type. 3. Despite the fact that the SiIV and CIV wind lines are variable in the Be stars, certain conclusions can be drawn from a statistical “snapshot” study such as this: a) The SiIV and CIV wind lines in the Be stars are correlated with both spectral type and luminosity class in the sense that the hottest stars have the strongest lines, and the giants and subgiants (at least for the B1—B3 stars) have stronger lines than the main-sequence stars. (see Fig. 1 and 2). b) The SiIV wind lines persist to spectral type B8 in both the Be stars and the standard stars but are stronger in the Be stars than in the standards for the earlier types (B1—B3). (See Fig. 1) c) The CIV wind lines persist to spectral type B8 in the Be stars but only to B3 in the standard stars. They are stronger in the Be stars than in the standards at all spectral types. (see Fig. 2). d) The equivalent widths of the SiIV and CIV wind lines are only very weakly correlated with vsini, if at all, but a threshold in vsini near 150 km s-1 (as found earlier by Grady et al.) exists below which no large equivalent widths of SiIV or CIV may be seen. Assuming that the Be stars are all rapid rotators, such a correlation is essentially a correlation with i and suggests that the winds from Be stars arise preferentially from the equatorial regions. The aforementioned conclusion is supported by a plot of SiIV 1394 centroid velocities versus vsini, which shows that stars with large velocity shifts also have large vsini, while those with unshifted lines all have vsini less than about 150 km s-1. Again, it seems that strong stellar winds are more likely to arise from equatorial than polar regions. e) Additional evidence that the winds are stronger for Be stars of early type comes from the SiIV and CIV centroid velocities, which are much larger for the hotter Be stars. The stars with the strongest lines also tend to have the largest velocity shifts, suggesting that winds with more mass are also faster moving. f) The SiIV and CIV lines in the standard stars, while they may be asymmetric, never show displaced line centers. g) The edge velocities for both the SiIV 1394 and the CIV 1548 lines increase with earlier spectral type in the Be stars (therefore, stronger winds in the hotter stars), and are considerably higher in the Be stars than in the standard stars, suggesting again that the winds are stronger in the Be stars. h) Shell stars have weaker CIV absorption and smaller edge and centroid velocities than other Be stars, suggesting that they have weaker winds. Since there is considerable evidence that these are stars with cool, low-velocity disks which are being viewed edge-on or nearly edge-on, the winds may be inhibited and modified by the denser material in the equatorial regions. 4. Mg II emission is detected in about half of the program Be stars with long wavelength IUE spectra, and seems not to be correlated with spectral type, vsini, or strength of the SiIV wind lines. Since the MgII emission presumably originates in the cool, low velocity envelope and since MgII emission also correlates with hydrogen Balmer emission in the Be stars, this suggests that there is no strong physical relationship between the stellar winds and the cool disk.

The young cluster NGC 2264 contains early type and non-emission stars, as well as T Tauri-like variables which populate the elevated main sequence (Walker 1956). The astrometric study of Vasilevskis et al . (1965) indicates a high probability of these variable stars being cluster members. The generally accepted view is that the stars which lie above the zero-age main sequence are still undergoing contraction. They exhibit infra-red, blue and ultra-violet excesses (Mendoza 1968, Smak 1964), and they are characterized by H α emission, the intensity of which is correlated with ultra-violet excess (Kuhi 1966). In the (U–B, B–V) diagram these stars occupy the same region as would the heavily reddened OB stars. It is necessary to distinguish between them and this can be achieved by the application of multi-colour (UBVRI) photometry. In the present paper UBV photometry of the stars of NGC 2264 has been extended to R and I magnitudes in Johnson's (1966) system, and the infra-red and ultra-violet excesses of the T Tauri-like stars have been studied. The UBV data have been taken from Walker (1956). The plates were taken with the 40/60/150 cm Schmidt telescope of the Royal Observatory, Edinburgh's outstation at Monte Porzio during the period from October 1968 to January 1969. For R and I magnitudes the emulsion/filter combinations used are Kodak IaE + Ilford colour filter 204 and Kodak IN+ Ilford colour filter 207 respectively. The photographic calibration has been achieved by using a pair of doubly refracting calcite plates in conjunction with a polaroid (Bruck et al . 1969). The limiting magnitudes are R = 14 m .0 and I = 13 m .0. No attempt has been made to search for variables. Variability of the known or suspected variables has been studied over the period of observations, and mean magnitudes have been derived from plates taken in the same interval of time. The standard error of photographic magnitudes due to different sources of errors including the variability of stars is ± 0 m .14 The separation of T Tauri-like stars from background early type stars which are heavily reddened can be achieved in the following way: The (U–V) and (V–I) colours can be predicted for the early type stars from the parameter Q defined as Q = (U–B)–0.70 (B–V) (Nandy 1968), which determines the spectral type and colour excesses E B–V , E V–R , E V–I . In the case of reddened early type stars the predicted colours would agree with the observed (V–R) and (V–I) colours. On the other hand, if the difference between the predicted and observed colours is much larger than the errors of observation, the stars are likely to be of later types with anomalous colours characteristic of T Tauri stars. For these stars, an additional parameter (V–R) 0 , which is the observed (V–R) colour corrected for reddening has been found necessary to estimate their spectral types, and infrared and ultraviolet excesses can be directly computed. The variables, in particular the H α emission stars in NGC 2264, exhibit infra-red and ultra-violet excesses, and there appears to be a positive correlation between them. These stars may be more luminous than indicated by their colour magnitude diagrams, and following Mendoza (1968) the mean ratio L total /L cm has been assumed to be 3. Masses and ages of these variables as faint as V = 15 m .0 have been computed on this assumption. It is found that their average age is 10 6 years, in agreement with the estimated age of the B stars of the cluster, and their masses range from one to two solar masses.

Aims. In the framework of the search for extrasolar planets and brown dwarfs around early-type stars, we present the results obtained for the F-type main-sequence star HD 60532 (F6V) with HARPS.

We present spectroscopy from 2.0 to 2.33 mum of 32 luminous young stellar objects (YSOs), which are presumed to be precursors of Herbig Ae/Be stars. From these stars, Br gamma, H(2), CO, He I, and Fe II were found in emission with detection rates of 97%, 34%, 22%, 9%, and 3%, respectively. We compare the spectral features with those of Herbig Ae/Be stars in the literature to investigate the spectral behavior of intermediate- to high-mass YSOs and to search for their relations to the spectral energy distributions (SEDs). H(2) emission is detected only in Class I SEDs with particularly large spectral indices. The detection of H(2) emission is related to the degree of the dispersal of circumstellar envelopes, where H(2) molecules are probably excited by shocks from outflows. On the other hand, Br gamma emission, which is generally thought to occur in stellar winds close to the stars, does not depend on the SEDs. This indicates that stellar wind from luminous YSOs does not change much from the embedded phase to the optically visible phase. CO emission is also independent of the SEDs, but the detection rate is much lower than that of Br gamma emission. Probably, more specific physical conditions regarding circumstellar disks and stellar radiation are necessary for CO emission to take place.

Numerical simulations of the three-dimensional structure and time evolution of stellar surface convection are now possible, at least for solar-type stars. Using the output from such simulations as sets of spatially and temporally varying model atmospheres, synthetic granulation images and spectral line profiles are computed, and compared to observations. Thus obtained disk-integrated data agree with observed lineshapes and bisector patterns in different stars, and also permit stellar rotation to be determined. Such simulations represent the first generation of models that are free from the classical ad hoc parameters of ‘mixing-length’, ‘micro-’ or ‘macro-turbulence’, parameters which in the past have characterized and limited stellar astrophysics.To infer the surface structure also in more exotic stars, simpler and parametrized models must still be used to interpret observed line asymmetries. Such models suggest that rapidly rising ‘granules’ cover only a small surface fraction on early-type stars, a situation opposite to that in solar-type ones, and one likely to affect magnetic fields and stellar activity. Theoretical challenges for the future include detailed modeling also of early-type, giant, and other non-solar type stars of different rotational velocities; the hydrodynamics of entire stellar envelopes (including the interaction with global oscillations); and the interaction with magnetic fields (including their generation). Greatly increased computing power will be needed for such detailed modeling throughout the Herzsprung-Russell diagram, possibly requiring custom-designed computers.Signatures of stellar granulation are primarily observed as asymmetries and wavelength shifts in photospheric absorption lines. Observational challenges include achieving sufficient spectral resolution to fully resolve such asymmetries; identifying granulation signatures throughout the HR-diagram (including the blended spectra of cool stars); observing how line asymmetries for a given spectral type depend on stellar rotational velocity, measuring wavelength shifts between groups of different lines in the same star, and between different stars; monitoring lineshift variations during stellar activity cycles; and ultimately high-resolution spectroscopy of spatially resolved granulation structures across stellar disks. The latter will require active optics on future very large telescopes, or the use of long-baseline optical interferometers.

On QbjeCtiVe-priSm photographs of three areas of the Taurus dark clouds, 40 faint stars with Ha emission were discovered. Slit spectrograms of 35 of these stars were examined for the purpose of finding spectral ,peculiarities which might be attributed to interaction between the stars and the surrounding clouds. Except for two stars of type Be which are probably not involved in the clouds and one peculiar A4e star, the stars are main-sequence stars of late spectral type and low luminosity. The stars are con- centrated in small regions in or near areas of great obscuration. If these stars are of normal luminosity and are at the distance of the clouds, their apparent magnitudes are generally too faint as a result of space absorption. The spectra show many of the characteristics previously found in the T Tauri variables. The hydrogen and Ca ii (H and K) bright lines are extremely strong in spectral types dGe-dM2.5e. Emission lines of He I, Fe I, Fe II, and [S ii] decrease in intensity in later spectral types in the two Taurus areas but are practic3lly absent in stars of the area in Orion. Related stars-A few stars with similar spectra, located in other regions of the sky are discussed for comparison with the Taurus stars. Radi~al velocities.-The approximate radial velocities from emission and absorption lines were meas- ured. The differences, emission minus absorption, seem to be greatest for the earlier types, which also show the strongest T Tauri characteristics

The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars. We propose three main groups to describe the observed variety in pulsating A-F type stars: gamma Dor, delta Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the gamma Dor and delta Sct range, also between 5 and 10 c/d, which is a challenge for the current models. We find indications for the existence of delta Sct and gamma Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the delta Sct and gamma Dor instability strips, and beyond. Non-variable stars seem to exist within the instability strips. The location of gamma Dor and delta Sct classes in the (Teff,logg)-diagram has been extended. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between gamma Dor and delta Sct stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of other pulsation mechanisms to supplement the kappa mechanism and convective blocking effect to drive hybrid pulsations.

We present a new millimeter survey of 23 pre-main-sequence stars in the Taurus-Auriga star-forming region. The main goal of the project was to identify circumstellar disks around stars of late spectral types (M {sub *} {approx}< 0.7 M {sub sun}). We used the IRAM Plateau de Bure Interferometer to obtain observations at 1.3 mm, 2.7 mm, and the CO J = 1-0 and J = 2-1 lines. We detected eight sources in continuum emission, with new detections of CIDA-1 and CIDA-8 at 1.3 mm. Overall, we find that circumstellar disks around stars with spectral types later than M2 tend to have smaller millimeter continuum fluxes than those around earlier type stars. We also present four detections of disks in their CO line emission (LkH{alpha} 358, GO Tau, Haro 6-13, IRAS 04385+2550). By mapping the rotation of the resolved CO emission from the circumstellar disks, we obtain estimates of the dynamical mass of the central stars.

Tidal dissipation in late-type stars is presently poorly understood and the study of planetary systems hosting hot Jupiters can provide new observational constraints to test proposed theories. We focus on systems with F-type main-sequence stars and find that the recently discovered system CoRoT-11 is presently the best suited for such a kind of investigation. A classic constant tidal lag model is applied to reproduce the evolution of the system from a plausible nearly synchronous state on the ZAMS to the present state, thus putting constraints on the average modified tidal quality factor <Q'_s> of its F6V star. Initial conditions with the stellar rotation period longer than the orbital period of the planet can be excluded on the basis of the presently observed state in which the star spins faster than the planet orbit. It is found that <Q'_s> is approximately between 4.0E+06 and 2.0E+07 if the system started its evolution on the ZAMS close to synchronization with an uncertainty related to the constant tidal lag hypothesis and the estimated stellar magnetic braking within a factor of about 5-6. Moreover, we discuss how the present value of Q'_s can be measured by a timing of the mid-epoch and duration of the transits as well as of the planetary eclipses to be observed in the infrared with an accuracy of about 0.5-1 s over a time baseline of about 25 yr. CoRoT-11 is an highly interesting system potentially allowing us a direct measure of the tidal dissipation in an F-type star as well as the detection of the precession of the orbital plane of the planet that provides us with an accurate upper limit for the obliquity of the stellar equator. If the planetary orbit has a significant eccentricity (e>0.05), it will be possible to detect also the precession of the line of the apsides and derive information on the Love number of the planet and its tidal quality factor.

We report on results obtained with theXMM-Newtonobservation of Feige 34 carried out in April 2018. This is the first spectroscopic X-ray observation of a compact and helium-poor hot subdwarf star. The source was detected at a flux levelfX = 3.4 × 10−14erg cm−2s−1in the energy range 0.2–3 keV, which implies an X-ray-to-bolometric flux ratiofX/fbol ≃ 10−6.5. The source spectrum can be described with the sum of two thermal-plasma components with subsolar abundances at temperatures of ≃0.3 and 1.1 keV. These properties are similar to what is observed in early-type main-sequence stars, where the X-ray emission is attributed to turbulence and shocks in the stellar wind. Therefore, the same phenomenon could explain the X-ray properties of Feige 34. However, it is not possible to reproduce the observed spectrum with a thermal-plasma model if the elemental abundances are fixed at the values obtained from the optical and UV spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are consistent with those expected from a young main-sequence star of late spectral type. Therefore, we discuss the possibility that the observed X-ray emission is due to the companion star of M0 spectral type, whose presence is suggested by the IR excess in the spectral energy distribution of Feige 34.

A number of massive stars of early type is found in X-ray binary systems. The catalog of Bradt et al. (1979) contains 21 sources optically identified with massive stars ranging in spectral type from 06 to B5 out of which 13 are (nearly) unevolved stars and 8 are supergiants. Single stars of this type generally show moderate to strong stellar winds. The X-rays in these binaries originate from accretion onto a compact companion (we restrict the discussion to this type of X-rays).We consider the compact star as a probe traveling through the stellar wind. This probe enables us to derive useful information about the mass outflow of massive stars.After presenting the basic data we derive an upper limit to mass loss rates of unevolved early type stars by studying X-ray pulsars. Next we consider theoretical predictions concerning the influence of X-rays on the stellar wind and compare these with the observations. Finally, using new data from IUE, we draw some conclusions about mass loss rates and velocity laws as derived from X-ray binaries.

More than 600 high resolution spectra of stars with spectral type F and later were obtained in order to search for signatures of differential rotation in line profiles. In 147 stars, the rotation law could be measured, 28 of them are found to be differentially rotating. Comparison to rotation laws in stars of spectral type A reveals that differential rotation sets in at the convection boundary in the HR-diagram; no star that is significantly hotter than the convection boundary exhibits the signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at vsini~100 km/s show extraordinarily strong absolute shear at short rotation periods around one day. It is suggested that this is due to their small convection zone depth and that it is connected to a narrow range in surface velocity. Detection frequencies of differential rotation were analyzed in stars with varying temperature and rotation velocity. Measurable differential rotation is more frequent in late-type stars and slow rotators. The strength of absolute shear and differential rotation are examined as functions of the stellar effective temperature and rotation period. The strongest shear is found at rotation periods between two and three days. In slower rotators, the strongest shear at a given rotation rate is given approximately by DOmega_max ~ P^{-1}. In faster rotators, alpha_max and DOmega_max diminish less rapidly. A comparison with differential rotation measurements in stars of later spectral type shows that F-stars exhibit stronger shear than cooler stars do, the upper boundary in absolute shear DOmega with temperature is consistent with the temperature scaling law found in Doppler Imaging measurements.