Zero-age Main-sequence Stars Research Articles

AHerschelstudy of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey

We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2 \times 2 \degr Galactic Plane fields centered at [l;b] = [30\degr; 0 \degr] and [l;b] = [59\degr; 0 \degr], respectively, and observed during the Science Demonstration Phase for the Herschel infrared Galactic Plane survey (Hi-GAL) Key-Project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 $\mu$m) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the l = 30\degr field, and 444 entries for the l = 59\degr field. The SED coverage has been augmented with ancillary data at 24 $\mu$m and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The Lbol/Menv ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the l = 59\degr field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30\degr field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm$^{-2}$ critical threshold for high-mass star formation.

A new open-source code for spherically symmetric stellar collapse to neutron stars and black holes

We present the new open-source spherically symmetric general-relativistic (GR) hydrodynamics code GR1D. It is based on the Eulerian formulation of GR hydrodynamics (GRHD) put forth by Romero–Ibáñez–Gourgoulhon and employs radial-gauge, polar-slicing coordinates in which the 3+1 equations simplify substantially. We discretize the GRHD equations with a finite-volume scheme, employing piecewise-parabolic reconstruction and an approximate Riemann solver. GR1D is intended for the simulation of stellar collapse to neutron stars and black holes and will also serve as a testbed for modeling technology to be incorporated in multi-D GR codes. Its GRHD part is coupled to various finite-temperature microphysical equations of state in tabulated form that we make available with GR1D. An approximate deleptonization scheme for the collapse phase and a neutrino-leakage/heating scheme for the postbounce epoch are included and described. We also derive the equations for effective rotation in 1D and implement them in GR1D. We present an array of standard test calculations and also show how simple analytic equations of state in combination with presupernova models from stellar evolutionary calculations can be used to study qualitative aspects of black hole formation in failing rotating core-collapse supernovae. In addition, we present a simulation with microphysical equations of state and neutrino leakage/heating of a failing core-collapse supernova and black hole formation in a presupernova model of a 40 M⊙ zero-age main-sequence star. We find good agreement on the time of black hole formation (within 20%) and last stable protoneutron star mass (within 10%) with predictions from simulations with full Boltzmann neutrino radiation hydrodynamics.

First stars. I. Evolution without mass loss

The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M > 100 Mo. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300 Mo galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10E-6 and 10E-9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low metallicity massive stars are hotter and more compact and luminous than their metal enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have an important influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.

The environs of the ultracompact HII region G45.45+0.06

G45.45+0.06 is an ultra-compact HII (UCHII) region extensively studied. It is known that G45.45+0.06 is embedded in a complex of UCHII regions, but up the date, the surrounding ISM in a larger spatial scale has not been analyzed. Using data from large-scale surveys: Two Micron All Sky Survey, GLIMPSE, MIPSGAL, MAGPIS and GRS, we performed a multiwavelength study of a region about 7' x 7'in the vicinity of G45.45+0.06. We found that the UCHII complex lies in a border of a larger (diameter of ~ 3') and fainter HII region, which is located at the same distance as G45.45+0.06, ~ 8 kpc. In this work, this larger HII region was called G45L. A good morphological correlation is observed between the PDRs and the molecular gas mapped in the 13CO J=1--0 and CS J=2--1 lines, suggesting that G45L may be collecting the molecular material. From a near- and mid-IR photometric study, we found three sources, likely O-type stars, that are possibly responsible for the creation of G45L. Additionally we found several YSO candidates lying preferently in the molecular shell that surrounds G45L. Our results confirm that the region southeastern the UCHII complex where G45.45+0.06 is embedded and eastern G45L is active in star formation. We suggest that G45L has been expanding during about 2 x 10^6 yr and could have triggered the formation of the zero-age main sequence stars that are ionizing the UCHII region G45.45+0.06. However we can not discard that both HII regions are coeval.

EVOLUTION OF MASSIVE PROTOSTARS WITH HIGH ACCRETION RATES

Formation of massive stars by accretion requires a high accretion rate of to overcome the radiation pressure barrier of the forming stars. Here, we study evolution of protostars accreting at such high rates by solving the structure of the central star and the inner accreting envelope simultaneously. The protostellar evolution is followed starting from small initial cores until their arrival at the stage of the Zero-Age Main-Sequence (ZAMS) stars. An emphasis is put on evolutionary features different from those with a low accretion rate of , which is presumed in the standard scenario for low-mass star formation. With the high accretion rate of , the protostellar radius becomes very large and exceeds 100 R☉. Unlike the cases of low accretion rates, deuterium burning hardly affects the evolution, and the protostar remains radiative even after its ignition. It is not until the stellar mass reaches ≃40 M☉ that hydrogen burning begins and the protostar reaches the ZAMS phase, and this ZAMS arrival mass increases with the accretion rate. These features are similar to those of the first star formation in the universe, where high accretion rates are also expected, rather than to the present-day low-mass star formation. At a very high accretion rate of >3 × 10−3 M☉ yr-1, the total luminosity of the protostar becomes so high that the resultant radiation pressure inhibits the growth of the protostars under steady accretion before reaching the ZAMS stage. Therefore, the evolution under the critical accretion rate 3 × 10−3 M☉ yr-1 gives the upper mass limit of possible pre-main sequence stars at ≃60 M☉. The upper mass limit of MS stars is also set by the radiation pressure onto the dusty envelope under the same accretion rate at ≃250 M☉. We also propose that the central source enshrouded in the Orion KL/BN nebula has effective temperature and luminosity consistent with our model and is a possible candidate for such protostars growing under the high accretion rate.

UNVEILING A COMPACT CLUSTER OF MASSIVE AND YOUNG STARS IN IRAS 17233-3606

We have analyzed sensitive high spatial resolution archival radio continuum data at 1.3, 2.0, 3.6, and 6.0 cm as well as the H2O maser molecular line data obtained using the Very Large Array (VLA) in its hybrid AB configuration toward the high-mass star-forming region IRAS 17233-3606 (G351.78-0.54). We find nine compact radio sources associated with this region, six of them are new radio detections. We discuss the characteristics of these sources based mostly on their spectral indices and find that most of them appear to be optically thin or thick ultra- and hyper-compact H II regions ionized by B zero-age main-sequence stars. Furthermore, in a few cases the radio emission may arise from optically thick dusty disks and/or cores; however more observations at different wavelengths are necessary to firmly confirm their true nature. In addition, we compared our centimeter maps with the mid-infrared images from the Spitzer Space Observatory GLIMPSE survey revealing a cluster of young protostars in the region together with multiple collimated outflows, some of which might be related to the compact centimeter objects. Finally, we find that one of these centimeter objects, VLA2d, is well centered in an apparent strong and compact north-south bipolar outflow traced by OH masers and we therefore suggest that this object is energizing the latter.

The pre-ZAMS nature of Mol160/IRAS 23385+6053 confirmed by Spitzer

Context. The formation of massive stars goes through phases that remain heavily obscured until the object is well along on the main sequence. The identification of massive YSOs in different evolutionary phases is therefore particularly difficult, and requires a statistical approach with large samples of candidate objects to determine the observational signatures of these different phases. Aims. A mandatory activity in this context is the identification and characterisation of all the phases that a massive forming YSO undergoes. It is of particular interest to verify the observability of the phase in which the object is rapidly accreting while not yet igniting the fusion of hydrogen that marks the arrival on the ZAMS. Methods. One of the candidate prototypical objects for this phase is Mol160/ΙRAS 23385+6053, which has been the subject of detailed studies that confirmed and strengthened the possibility that this massive YSO may be in a pre-hot core stage. We further investigate this issue by means of Spitzer imaging and spectroscopy in the 5-70 μm range. Results. The dense core of Mol160/ΙRAS 23385+6053, which up to now had only been detected at submillimeter and millimeter wavelengths, with only upper limits below 20 pm, has been revealed for the first time at 24 and 70 μm by Spitzer. These observations confirm the earlier assumptions that this object is dominant at far-IR wavelengths. The complete 24 μm-3.4 mm continuum cannot be fitted with a standard model of a zero-age main-sequence (ZAMS) star embedded in an envelope. A simple greybody fit yields a mass of 220 M ⊙ . The luminosity is slightly in excess of 3000 L ⊙ , which is a factor of 5 less than previous estimates when only IRAS fluxes were available between 20 and 100 μm. The source is under-luminous by the same factor with respect to UCHII regions or hot-cores of similar circumstellar mass, and simple models show that this is compatible with an earlier evolutionary stage. Spectroscopy between 5-40 μm shows that the physical conditions are typical of a photo-dissociated or photo-ionized region. The required UV illumination can be provided by some of the other sources revealed at A ≤ 24 μm in the same star-forming region, that can be plausibly modeled as moderately embedded intermediate-mass ZAMS stars. Conclusions. Our results strengthen the suggestion that the central core in Mol160/IRAS 23385+6053 is a massive YSO actively accreting from its circumstellar envelope and that it has not yet begun hydrogen fusion.

Third Order Effect of Rotation on Stellar Oscillations of a B Star

The present paper aims at investigating the effect of rotation up to third order in the angular velocity of a star on p and g modes, based on the formalism developed by Soufi et al. (1998). Our ultimate goal is the study of oscillations of $\beta$ Cephei stars which are often rapidly rotating stars. Our study shows that the third-order perturbation formalism presented by Soufi et al. (1998) must be corrected for some missing terms and some misprints in the equations. As a first step in our study of $\beta$ Cephei stars, we quantify by numerical calculations the effect of rotation on the oscillation frequencies of a uniformly rotating zero-age main-sequence star with 12~$M_\odot$. For an equatorial velocity of 100~$\rm km \, s^{-1}$, it is found that the second- and third-order corrections for $(l,m)=(2,2)$, for instance, are of order of 0.01$\%$ of the frequency for radial order $n=6$ and reaches up to 0.5$\%$ for $n=14$.

The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg with the Hobby-Eberly Telescope (HET). Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K_{K} = 162.5 +/- 1.0 km/s and q= M_{K} /M_{wd} = 0.685 +/- 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 +/- 0.075 of the total flux at 5500 angstroms. Taking the weighted average of our results with previously published results obtained using similar techniques, we find <K_{K}> = 163.7 +/- 0.7 km/s and <q> = 0.683 +/- 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45 deg. <= i <= 56 deg. The derived masses of the K star and white dwarf are M_{K} = 0.55 +/- 0.13 M_sun and M_{wd} = 0.81 +/- 0.19 M_sun, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main-Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted.

High-resolution X-ray spectroscopy of T Tauri stars in the Taurus-Auriga complex

Context. Di erences have been reported between the X-ray emission of accreting and non-accreting stars. Some observations have suggested that accretion shocks could be responsible for part of the X-ray emission in classical T Tauri stars (CTTS). Aims. We present high-resolution X-ray spectroscopy for nine pre-main sequence stars in order to test the proposed spectroscopic di erences between accreting and non-accreting pre-main sequence stars. Methods. We used X-ray spectroscopy from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. We interpret the spectra using optically thin thermal models with variable abundances, together with an absorption column density. For BP Tau and AB Aur we derive electron densities from the Ovii triplets. Results. Using the Ovii/Oviii count ratios as a diagnostic for cool plasma, we find that CTTS display a soft excess (with equivalent electron temperatures of 2:5 3 MK) when compared with WTTS or zero-age main-sequence stars. Although the Ovii triplet in BP Tau is consistent with a high electron density (3:4 10 11 cm 3 ), we find low density for the accreting Herbig star AB Aur (ne < 10 10 cm 3 ). The element abundances of accreting and non-accreting stars are similar. The Ne abundance is found to be high (4-6 times the Fe abundance) in all K and M-type stars. In contrast, for the three G-type stars (SU Aur, HD 283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4-0.8 times solar photospheric values) compared to later-type stars. Conclusions. Adding the results from our sample to former high-resolution studies of T Tauri stars, we find a soft excess in all accreting stars, but in none of the non-accretors. On the other hand, high electron density and high Ne/Fe abundance ratios do not seem to be present in all accreting pre-main sequence stars.

An XMM—Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr

Abstract We report on XMM—Newton observations of the young open cluster NGC 2547 which allow us to characterize coronal activity in solar-type stars, and stars of lower mass, at an age of 30 Myr. X-ray emission is seen from stars at all spectral types, peaking among G stars at luminosities (0.3-3 keV) of Lx≃ 1030.5 erg s−1 and declining to Lx≤ 1029.0 erg s−1 among M stars with masses ≥0.2 M⊙. Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances of Z≃ 0.3. The G- and K-type stars of NGC 2547 follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most of the solar-type stars in NGC 2547 exhibit saturated or even supersaturated X-ray activity levels. The median levels of Lx and Lx/Lbol in the solar-type stars of NGC 2547 are very similar to those in T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with Lx, Lx/Lbol and surface X-ray flux. The most active solar-type stars in NGC 2547 have coronal temperatures intermediate between those in the ONC and the most active older zero-age main-sequence (ZAMS) stars. We show that simple scaling arguments predict higher coronal temperature in coronally saturated stars with lower gravities. A number of candidate flares were identified among the low-mass members and a flaring rate [for total flare energies (0.3-3 keV) &amp;gt; 1034 erg] of one every 350+350−120 ks was found for solar-type stars, which is similar to rates found in the ONC and Pleiades. Comparison with ROSAT High Resolution Imager (HRI) data taken 7 yr earlier reveals that only 10-15 per cent of solar-type stars or stars with Lx &amp;gt; 3 × 1029 erg s−1 exhibit X-ray variability by more than a factor of 2. This is comparable with clusters of similar age but less than in both older and younger clusters. The similar median levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr.

Optical spectroscopy of X-Mega targets in the Carina nebula - VI. FO 15: a new O-type double-lined eclipsing binary

We report the discovery of a new O-type double-lined spectroscopic binary with a short orbital period of 1.4 d. We find the primary component of this binary, FO 15, to have an approximate spectral type 05.5Vz, i.e. a zero-age main-sequence (ZAMS) star. The secondary appears to be of spectral type 09.5V. We have performed a numerical model fit to the public All Sky Automated Survey photometry, which shows that FO 15 is also an eclipsing binary. We find an orbital inclination of ∼80°. From a simultaneous light curve and radial velocity solution, we find the masses and radii of the two components to be 30 ± 1 and 16 ± 1M ⊙ and 7.5 ± 0.5 and 5.3 ± 0.5 R ⊙ . These radii, and hence also the luminosities, are smaller than those of normal O-type stars, but similar to recently bom ZAMS O-type stars. The absolute magnitudes derived from our analysis locate FO 15 at the same distance as η Carinae. From Chandra and XMM X-ray images, we also find that there are two close X-ray sources: one coincident with FO 15 and the other one without optical counterpart. The latter seems to be a highly variable source, presumably due to a pre-main-sequence stellar neighbour of FO 15.

Infrared and Radio Study of Star‐forming Regions Associated with IRAS 19111+1048 and IRAS 19110+1045

A multiwavelength study of the star forming regions associated with IRAS 19111+1048 and IRAS 19110+1045 has been carried out. These have been simultaneously mapped in two far infrared bands at lambda_eff ~ 130 and 200 micron with ~1' angular resolution using the TIFR 1-m balloon borne telescope. The radio emission from the ionised gas of these regions has been imaged at 1280, 610 and 325 MHz using the Giant Metrewave Radio Telescope, India. A total of 20 compact radio sources have been detected from the high resolution radio map of IRAS 19111+1048 at 1280 MHz. Assuming these sources to represent exciting zero age main sequence (ZAMS) stars, the initial mass function is found to be quite steep, with the power law index of 5.3+-0.5 for the mass range 14 < m/M_sun < 33. The spectral types of the ZAMS stars inferred independently from the radio and NIR measurements match very well for a good fraction of the radio sources having NIR counterparts. For IRAS 19110+1045 region, seven radio sources have been detected of which two are associated with deeply embedded 2MASS objects. Self consistent radiative transfer modelling aimed at extracting important physical and geometrical details of the two IRAS sources has been carried out. A uniform density distribution of dust and gas is implied for both the sources. The extents of ionised gas, number of ZAMS stars, presence of deeply embedded sources and lower value of L/M for the cloud, support the youth of IRAS 19110+1045 vis-a-vis its neighbour, IRAS 19111+1048, consistent with earlier studies.

The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin‐up

The zero-age main-sequence star AB Dor and the K dwarf component of the V471 Tau close binary have essentially identical rotation rates and spectral types. An analysis of their high-resolution Chandra X-ray spectra reveals remarkably similar coronal characteristics in terms of both temperature structure and element abundances. Both stars show depletions of low first ionization potential (FIP) elements by factors of ~3, with higher FIP elements showing more mild depletions. No evidence for enhancements of very low FIP (<7 eV) elements, such as Na, Al, and Ca, as compared to other low-FIP elements, was found. The abundance anomaly pattern for AB Dor and V471 Tau is similar to, although less extreme than, the abundance anomalies exhibited by active RS CVn-type binaries. While we find statistically significant structure in the underlying differential emission measure distributions of these stars over narrow temperature intervals, this structure is strongly dependent on the lines used in the analysis and is probably spurious. On the basis of their X-ray similarities, we conclude that the exact evolutionary state of a star has little effect on coronal characteristics and that the parameters that dominate coronal structure and composition are simply the rotation rate and spectral type.

Coronal properties of G-type stars in different evolutionary phases

We report on the analysis of XMM-Newton observations of three G-type stars in very different evolutionary phases: the weak-lined T Tauri star HD 283572, the Zero Age Main Sequence star EK Dra and the Hertzsprung-gap giant star 31 Com. They all have high X-ray luminosity (∼10 31 erg s -1 for HD 283572 and 31 Com and ∼10 30 erg s -1 for EK Dra). We compare the Emission Measure Distributions (EMDs) of these active coronal sources, derived from high-resolution XMM-Newton grating spectra, as well as the pattern of elemental abundances vs. First lonzation Potential (FIP). We also perform time-resolved spectroscopy of a flare detected by XMM from EK Dra. We interpret the observed EMDs as the result of the emission of ensembles of magnetically confined loop-like structures with different apex temperatures. Our analysis indicates that the coronae of HD 283572 and 31 Com are very similar in terms of dominant coronal magnetic structures, in spite of differences in the evolutionary phase, surface gravity and metallicity. In the case of EK Dra the distribution appears to be slightly flatter than in the previous two cases, although the peak temperature is similar.

LkHα 101 and the Young Cluster in NGC 1579

The central region of the dark cloud L1482 is illuminated by LkHα 101, a heavily reddened (AV ≈ 10 mag) high-luminosity (≥8 × 103 L⊙) star having an unusual emission-line spectrum plus a featureless continuum. About 35 much fainter (mostly between R = 16 and >21) Hα emitters have been found in the cloud. Their color-magnitude distribution suggests a median age of about 0.5 Myr, with considerable dispersion. There are also at least five bright B-type stars in the cloud, presumably of about the same age; none show the peculiarities expected of HAeBe stars. Dereddened, their apparent V magnitudes lead to a distance of about 700 pc. Radio observations suggest that the optical object LkHα 101 is in fact a hot star surrounded by a small H II region, both inside an optically thick dust shell. The level of ionization inferred from the shape of the radio continuum corresponds to a Lyman continuum luminosity appropriate for an early B-type zero-age main-sequence star. The V - I color is consistent with a heavily reddened star of that type. However, the optical spectrum does not conform to this expectation: the absorption lines of an OB star are not detected. Also, the [O III] lines of an H II region are absent, possibly because those upper levels are collisionally deexcited at high densities. There are several distinct contributors to the optical spectrum of LkHα 101. The Hα emission line is very strong, with wings extending to about ±1700 km s-1, which could be produced by a thin overlying layer of hot electron scatterers. There is no sign of P Cygni type mass ejection. Lines of Si II are narrower, while the many Fe II lines are still narrower and are double with a splitting of about 20 km s-1. Lines of [Fe II], [O I], and [S II] are similarly sharp but are single, at the same velocity as the Fe II average. Work by Tuthill et al. allowed the inference, from K-band interferometry, that the central source is actually a small horseshoe-shaped arc about 005 (35 AU) across. A tipped annulus of that size in rotation about a 15 M⊙ star would produce double spectrum lines having about the splitting observed for Fe II. The totality of observational evidence encourages the belief that LkHα 101 is a massive star caught in an early evolutionary state.

Magnetic fields in massive stars: dynamics and origin

Evidence continues to accumulate in favour of the presence of magnetic fields on the surfaces of massive stars. Some authors hypothesize that such fields originate in a dynamo in the convective core, with buoyancy bringing flux tubes to the surface. Here we show that, when realistic stellar models are used, this ‘core dynamo hypothesis’ encounters a serious difficulty: in order for surface fields to originate in a core dynamo, the core must create magnetic fields that are much stronger than equipartition values. As an alternative hypothesis for the origin of magnetic fields in OB stars, we suggest that a dynamo is operating in shear-unstable gas in the radiative stellar envelope. Using a recently developed code for the evolution of rotating stars, we find that in a 10-M⊙ zero-age main sequence (ZAMS) star with a typical rotation period, more than 90 per cent of the volume of the star is subject to shear instability. We have recently proposed that dynamo operation in shear-unstable gas helps to explain certain properties of mass loss in cool giants. Here, in the context of hot stars, we show that fields which originate in the shear-unstable regions of the envelope can reach the surface without violating the constraints of equipartition. Such fields rise to the surface on time-scales which are short compared to main-sequence lifetimes.

ROSATView ofHipparcosF Stars

For a set of 11,900 F stars that have uvby photometry and are in the Hipparcos catalog, we have found 1980 X-ray counterparts (~17%) in the ROSAT All-Sky Survey (RASS). In this paper we analyze X-ray properties of these ROSAT F stars and compare that sample with the entire set of our F stars. The comparison has shown that the ROSAT F stars are on average younger than stars in the whole set, consistent with previous studies that indicate lower X-ray emission in older stars. However, for the ROSAT stars only, the age-X-ray activity relation has proved to be more complicated, with a group of strong X-ray emitters being the oldest rather than the youngest stars. That group determines the shape of the overall relationship between the kinematics and X-ray luminosity. The relationship is in the sense that the mean tangential velocity and velocity dispersion are higher for stars that are more luminous at the X-ray wavelengths, implying that these stars are on average the oldest rather than the youngest. The group of strongest X-ray emitters, log LX > 30.4, which constitutes only a small fraction of the sample stars, ~2.8%, has been found to include a variety of stellar types. It is dominated by very young objects, followed by RS CVn-type stars and other active binaries. As to the sample as a whole, it seems to consist of at least three distinct groups of F stars that differ in their X-ray luminosity and evolution of their X-ray emission. The first one is represented by normal zero-age main-sequence (ZAMS) and post-ZAMS stars whose X-ray emission declines with age, just as expected in the conventional picture of stellar X-ray evolution (roughly half of the sample). The second group consists of strong X-ray emitters that appear to be very young, possibly pre-main-sequence stars. These stars are characterized by high metallicity, high temperature, and high overluminosity (the latter indicating that the star's optical luminosity is too high for the star's temperature and surface gravity). Many of them are located in the distance range ~130-200 pc, near the known regions of ongoing star formation or OB associations. The third group also consists of strong X-ray emitters. However, those are very old rather than very young stars, and they happen to be extremely overluminous as well. Their strong X-ray emission implies that many F stars are capable of increasing rather than decreasing their X-ray luminosity as they get older, thus challenging the idea that X-ray emission continuously declines with age. Most of these old X-ray emitters have significantly evolved away from the ZAMS. If they increase their outer convective zone as they evolve, they would probably be able to maintain a high level of coronal activity, and hence enhanced X-ray emission, despite decaying rotation. Other relationships found for the ROSAT F stars include the correlation of X-ray luminosity with hardness ratio, optical absolute magnitude, effective temperature, overluminosity, metallicity, and reddening. There is also a difference between binary and single stars, with X-ray luminosity being on average slightly higher for unresolved binaries. There is a distinct difference between the groups of soft and hard X-ray emitters in our sample: while the soft sources show a significant correlation between the X-ray luminosity and hardness ratio, no such correlation is seen for hard sources. The tendency for X-ray luminosity to be higher for harder sources implies that stars more luminous in the X-rays have a hotter corona. This tendency breaks up at large hardness values, which may be due to coronal saturation.

X‐Ray Modeling of Very Young Early‐Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications

The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of ~3 × 105 yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 A of three of its X-ray-brightest members (Θ1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe XVII to Fe XXV ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of Θ1 Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km s-1 (all widths are stated as half-width at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s-1. The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK for Θ1 Ori A. The high-temperature emissivity contributes over 70% to the total X-ray flux. None of the lines are resolved for Θ1 Ori A and E with a confidence limit of 160 km s-1. The influence of the strong UV radiation field on the forbidden line in the He-like triplets allows us to set an upper limit on distance of the line-emitting region from the photosphere. The bulk of the X-ray emission cannot be produced by shock instabilities in a radiation-driven and are likely the result of magnetic confinement in all three stars. Although confinement models cannot explain all the results, the resemblance of the unresolved lines and of the DEM with recent observations of active coronae in II Peg and AR Lac during flares is quite obvious. Thus we speculate that the X-ray production mechanism in these stars is similar, with the difference that the Orion stars may be in a state of almost continuous flaring driven by the wind. We clearly rule out major effects due to X-rays from a possible companion. The fact that all three stars appear to be magnetic and are near zero age on the main sequence also raises the issue of whether the Orion stars are simply different or whether young massive stars enter the main sequence carrying significant magnetic fields. The ratio log LX/Lbol using the wind component of the spectrum is -7 for the Trapezium stars, consistent with the expectation from O stars. This suggests that massive ZAMS stars generate their X-ray luminosities like normal O stars and magnetic confinement provides an additional source of X-rays.

Field zero-age main-sequence stars in the solar neighbourhood: where have they come from?

In the course of an all-sky survey for young stars in the solar neighbourhood, we have found a tight kinematic group of 10 F-G type zero-age main-sequence stars in the field. Here we discuss the origin of these stars. Backtracking the space motions of these stars, we argue that likely candidates for the parent association are the Perseus OB3 (Per OB3), Upper Centaurus-Lupus (IJCL) and Lower Centaurus-Crux (LCC) associations, and that we are witnessing the ongoing diffusion of (at least one of) these associations into the field.