Stellar Birthline Research Articles

Characterizing the X-Ray Emission of Intermediate-mass Pre-main-sequence Stars

Abstract We use X-ray and infrared observations to study the properties of three classes of young stars in the Carina Nebula: intermediate-mass (2–5 M ⊙) pre-main-sequence stars (IMPS; i.e., intermediate-mass T Tauri stars), late-B and A stars on the zero-age main sequence (AB), and lower-mass T Tauri stars (TTS). We divide our sources among these three subclassifications and further identify disk-bearing young stellar objects versus diskless sources with no detectable infrared (IR) excess emission using IR (1–8 μm) spectral energy distribution modeling. We then perform X-ray spectral fitting to determine the hydrogen-absorbing column density (N H ), absorption-corrected X-ray luminosity (L X ), and coronal plasma temperature (kT) for each source. We find that the X-ray spectra of both IMPS and TTS are characterized by similar kT and N H , and on average L X /L bol ∼ 4 × 10−4. IMPS are systematically more luminous in X-rays (by ∼0.3 dex) than all other subclassifications, with median L X = 2.5 × 1031 erg s−1, while AB stars of similar masses have X-ray emission consistent with TTS companions. These lines of evidence converge on a magnetocoronal flaring source for IMPS X-ray emission, a scaled-up version of the TTS emission mechanism. IMPS therefore provide powerful probes of isochronal ages for the first ∼10 Myr in the evolution of a massive stellar population, because their intrinsic, coronal X-ray emission decays rapidly after they commence evolving along radiative tracks. We suggest that the most luminous (in both X-rays and IR) IMPS could be used to place empirical constraints on the location of the intermediate-mass stellar birth line.

EXors and the stellar birthline

We assess the evolutionary status of EXors. These low-mass, pre-main-sequence stars repeatedly undergo sharp luminosity increases, each a year or so in duration. We place into the HR diagram all EXors that have documented quiescent luminosities and effective temperatures, and thus determine their masses and ages. Two alternate sets of pre-main-sequence tracks are used, and yield similar results. Roughly half of EXors are embedded objects, i.e., they appear observationally as Class I or flat-spectrum infrared sources. We find that these are relatively young and are located close to the stellar birthline in the HR diagram. Optically visible EXors, on the other hand, are situated well below the birthline. They have ages of several Myr, typical of classical T Tauri stars. Judging from the limited data at hand, we find no evidence that binarity companions trigger EXor eruptions; this issue merits further investigation. We draw several general conclusions. First, repetitive luminosity outbursts do not occur in all pre-main-sequence stars, and are not in themselves a sign of extreme youth. They persist, along with other signs of activity, in a relatively small subset of these objects. Second, the very existence of embedded EXors demonstrates that at least some Class I infrared sources are not true protostars, but very young pre-main-sequence objects still enshrouded in dusty gas. Finally, we believe that the embedded pre-main-sequence phase is of observational and theoretical significance, and should be included in a more complete account of early stellar evolution.

EPISODIC ACCRETION AT EARLY STAGES OF EVOLUTION OF LOW-MASS STARS AND BROWN DWARFS: A SOLUTION FOR THE OBSERVED LUMINOSITY SPREAD IN H-R DIAGRAMS?

We present evolutionary models for young low mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion ($\mdot \ge 10^{-4} \msolyr$) followed by longer quiescent phases ($\mdot < 10^{-6} \msolyr$) can explain the observed luminosity spread in HR diagrams of star forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant $\te$. The best agreement with the observed luminosity scatter is obtained if most of the accretion shock energy is radiated away. The obtained luminosity spread at 1 Myr in the HR diagram is equivalent to what can be misinterpreted as a $\sim$ 10 Myr age spread for non-accreting objects. We also predict a significant spread in radius at a given $\te$, as suggested by recent observations. These calculations bear important consequences on our understanding of star formation and early stages of evolution and on the determination of the IMF for young ($\le$ a few Myr) clusters. Our results also show that the concept of a stellar birthline for low-mass objects has no valid support.

Infrared Spectra of Young Stars Embedded in the R Coronae Australis Cloud

ABSTRACTInfrared spectra are presented for a magnitude-limited (K < 12.5 mag) sample of stellar objects observed toward the R Coronae Australis molecular core. These spectra, which are used to identify young stellar objects in the cloud, include the wavelengths of emission lines from [Fe II] and H 2, four of the Brackett series lines, the CO bandheads, as well as photospheric absorption lines of Al, Na, Mg, Si, and Ca. For a subset of the sample, the spectra are compared to those of infrared spectral standard stars to derive spectral types and luminosity classes. By comparing their placement in a Hertzsprung-Russell diagram with theoretical pre–main-sequence tracks and isochrones, we estimate the stellar masses and assess the evolutionary states of the members of this young aggregate. All of the sources classified via near-IR spectroscopy have masses in the range 0.2–2.5 M ⊙. The locus of points in the H-R diagram is lower than observed for other embedded clusters (e.g., NGC 2024 and the ρ Oph core), suggesting either a more advanced evolutionary state or a difference in the intrinsic stellar birthline for very young clusters. We discuss the implications of our results for the shape of the initial mass function of the embedded young cluster and the star-forming history of the cloud.

The evolution of ‘the moment of inertia’ of stars

AbstractObservations of the rotation periods of cool open cluster stars display a distinctive dichotomy when plotted against stellar mass/color. Other measures of stellar activity are also known to be dependent on stellar mass and structure, especially the onset and characteristics of convection zones. One proposal for understanding the observed rotation period dichotomy suggested dependencies on the moment of inertia of either the whole star or that of only the outer convection zone (Barnes 2003).The moment of inertia of stars with the mass between 0.1Msun and 3.0Msun have been calculated using a version of Yale Stellar evolution code (aka YREC). Each star has been evolved from stellar birthline to the onset of the core He burning. For easy comparison to observations, we have calculated the isochrones of these quantities as well as the convective turnover time, of interest to the activity community.

Laser Guide Star Adaptive Optics Integral Field Spectroscopy of a Tightly Collimated Bipolar Jet from the Herbig Ae Star LkHα 233

We have used the integral field spectrograph OSIRIS and laser guide star adaptive optics at Keck Observatory to obtain high angular resolution (0.06''), moderate spectral resolution (R 3800) images of the bipolar jet from the Herbig Ae star LkHα 233, seen in near-IR [Fe II] emission at 1.600 and 1.644 μm. This jet is narrow and tightly collimated, with an opening angle of only 9°, and has an average radial velocity of ~100 km s-1. The jet and counterjet are asymmetric, with the redshifted jet much clumpier than its counterpart at the angular resolution of our observations. The observed properties are in general similar to jets seen around T Tauri stars, although it has a relatively large mass flux of 1.2 ± 0.3 × 10-7 M☉ yr-1, near the high end of the observed mass flux range around T Tauri stars. We also spatially resolve an inclined circumstellar disk around LkHα 233, which obscures the star from direct view. By comparison with numerical radiative transfer disk models, we estimate the disk midplane to be inclined i = 65° ± 5° relative to the plane of the sky. Since the star is seen only in scattered light at near-infrared wavelengths, we detect only a small fraction of its intrinsic flux. Because previous estimates of its stellar properties did not account for this, either LkHa 233 must be located closer than previously believed, or its true luminosity must be greater than previously supposed, consistent with its being a ~4 M☉ star near the stellar birth line.

PRE-MAIN SEQUENCE EVOLUTIONS OF SOLAR ABUNDANCE LOW MASS STARS

We present the Pre-Main Sequence (PMS) evolutionary tracks of stars with <TEX>$0.065{\sim}5.0M_{\odot}$</TEX>. The models were evolved from the PMS stellar birthline to the onset of hydrogen burning in the core. The convective turnover timescales which enables an observational test of theoretical model, particulary in the stellar dynamic activity, are also calculated. All models have Sun-like metal abundance, typically considered as the stars in the Galactic disk and the star formation region of Population I star. The convection phenomenon is treated by the usual mixing length approximation. All evolutionary tracks are available upon request.

B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?

Projected rotational velocities (v sin i) have been measured for 216 B0–B9 stars in the rich, dense h and χ Persei double cluster and compared with the distribution of rotational velocities for a sample of field stars having comparable ages (t ~ 12–15 Myr) and masses (M ~ 4–15 M⊙). For stars that are relatively little evolved from their initial locations on the zero-age main sequence (ZAMS) (those with masses M ~ 4–5 M⊙), the mean v sin i measured for the h and χ Per sample is slightly more than 2 times larger than the mean determined for field stars of comparable mass, and the cluster and field v sin i distributions differ with a high degree of significance. For somewhat more evolved stars with masses in the range 5–9 M⊙, the mean v sin i in h and χ Per is 1.5 times that of the field; the v sin i distributions differ as well, but with a lower degree of statistical significance. For stars that have evolved significantly from the ZAMS and are approaching the hydrogen exhaustion phase (those with masses in the range 9–15 M⊙), the cluster and field star means and distributions are only slightly different. We argue that both the higher rotation rates and the pattern of rotation speeds as a function of mass that differentiate main-sequence B stars in h and χ Per from their field analogs were likely imprinted during the star formation process rather than a result of angular momentum evolution over the 12–15 Myr cluster lifetime. We speculate that these differences may reflect the effects of the higher accretion rates that theory suggests are characteristic of regions that give birth to dense clusters, namely, (1) higher initial rotation speeds; (2) higher initial radii along the stellar birth line, resulting in greater spin-up between the birth line and the ZAMS; and (3) a more pronounced maximum in the birth line radius-mass relationship that results in differentially greater spin-up for stars that become mid- to late-B stars on the ZAMS.

Stellar Rotation in Young Clusters: The First 4 Million Years

To investigate what happens to angular momentum during the earliest observable phases of stellar evolution, we searched the literature for periods (P), projected rotational velocities (v sin i), and supporting data on K5–M2 stars (corresponding to masses 0.25–1 M⊙) from the Orion Nebula Cluster and environs, ρ Ophiuchi, TW Hydra, Taurus-Auriga, NGC 2264, Chamaeleon, Lupus, and η Chamaeleonis. We combine these measures of rotation with the stellar R (as determined from Lbol and Teff) to compare the data with two extreme cases: conservation of stellar angular velocity and conservation of stellar angular momentum. Analysis of the P data set suggests that the frequency distribution of periods among the youngest and oldest stars in the sample is indistinguishable, while the v sin i data set reveals a decrease in mean v sin i as a function of age. Both results suggest that a significant fraction of all pre–main-sequence (PMS) stars must evolve at nearly constant angular velocity during the first ~3–5 Myr after they begin their evolution down the convective tracks. Hence, the angular momenta of a significant fraction of pre–main-sequence (PMS) stars must be tightly regulated during the first few million years after they first become observable. This result seems surprising at first glance, because observations of young main-sequence stars reveal a population (30%–40%) of rapidly rotating stars that must begin to spin up at ages t 5 Myr. To determine whether these apparently contradictory results are reconcilable, we use simple models along with our data set to place limits on (1) the fraction of PMS stars that must be regulated, and (2) the complementary fraction that could spin up as a function of time but escape statistical detection given the broad distribution of stellar rotation rates. These models include (1) instantaneous release at the stellar birthline of a given fraction of stars, with the remaining fraction regulated for 10 Myr; (2) all stars regulated initially, with the released fraction varying linearly with time, and timescales for release of half the stars varying from 0.5 to 5 Myr (i.e., all released by 1 to 10 Myr); and (3) a hybrid model that invokes assumptions (1) and (2). In all cases, we find that a modest population (30%–40%) of PMS stars could be released within the first 1 Myr and still produce period distributions statistically consistent with the observed data. This population is large enough to account for the rapid rotators observed among young main-sequence stars of comparable mass. The limits placed by our models on the fraction of regulated and released stars as a function of time are also consistent with the lifetime of accretion disks as inferred from near-IR excesses, and hence with the hypothesis that disk locking accounts for rotation regulation during early PMS phases.

The Evolution of Angular Momentum of Intermediate Mass Stars: From the Birthline to the Main Sequence

Measurements of stars in the Orion OB association show that there is a continuous power law relationship between specific angular momentum (J/M) and mass (M) for stars on convective tracks having masses in the range ~0.5 to ~3 M⊙; this power law extends smoothly into the domain of more massive stars on the ZAMS. If we assume that stars are “locked” to circumstellar accretion disks via their magnetic fields until they are deposited on the stellar birthline, we can account for the observed slope and zero point of the power law fit to the upper envelope of the observed J/M vs M distribution.Pre-main sequence stars with M&lt;2 M⊙ on radiative tracks do not follow the power law relationship. A sharp decrease in J/M with decreasing mass has been recognized for more than 30 years for older field stars, but remarkably is seen already among our Orion sample of stars that are only a few million years old. We show that this break in the power law is a consequence of loss of angular momentum on convective tracks, combined with core-envelope decoupling at the time of the transition from the convective to radiative tracks.

The Y 2 Isochrones for α‐Element Enhanced Mixtures

We present a new set of isochrones in which the effect of the alpha-element enhancement is fully incorporated. These isochrones are an extension of the already published set of YY Isochrones (Yi et al. 2001: Paper 1), constructed for the scaled-solar mixture. As in Paper 1, helium diffusion and convective core overshoot have been taken into account.The range of chemical compositions covered is 0.00001 < Z < 0.08. The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core. The age range of the full isochrone set is 0.1 -- 20 Gyr, while younger isochrones of age 1 -- 80 Myr are also presented up to the main-sequence turn-off. Combining this set with that of Paper 1 for scaled-solar mixture isochrones, we provide a consistent set of isochrones which can be used to investigate populations of any value of alpha-enhancement. We confirm the earlier results of Paper 1 that inclusion of alpha-enhancement effects further reduces the age estimates of globular clusters by approximately 8 percent if [alpha/Fe]=+0.3. It is important to note the metallicity dependence of the change in age estimates (larger age reductions in lower metallicities). This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations. The isochrone tables, together with interpolation routines have been made available via internet; http://www.astro.yale.edu/demarque/yyiso.html http://www-astro.physics.ox.ac.uk/~yi/yyiso.html http://csaweb.yonsei.ac.kr/~kim/yyiso.html

The Star Formation History of Trumpler 14 and Trumpler 16

H-R diagrams are presented for the very young galactic clusters Trumpler 14 and Trumpler 16, which are the two most populous clusters in the region of vigorous star formation surrounding η Carinae. Point spread function photometry of UBV CCD images is presented down to V ≈ 19 for over 560 stars in Tr 16 and 290 stars in Tr 14. We have also obtained similar data for a local background field. After determining cluster membership through proper motions from a previous work, we find that the reddening of cluster members is significantly lower than that of the local background stars. Thus, we are able to use individual reddenings to identify likely members at far deeper levels than possible with proper motions. This work has revealed a significant population of pre-main-sequence (PMS) stars in both clusters. The location of the PMS stars in the H-R diagram indicates that the theoretical stellar birthline of Palla & Stahler follows the locus of stars far better than that of Beech & Mitalas. Comparison with both pre- and post-main-sequence isochrones also reveals that although intermediate-mass stars have been forming continuously over the last 10 Myr, the high-mass stars formed within the last 3 Myr. There is no evidence that the formation of the intermediate-mass stars was truncated by the formation of the high-mass stars.

Rotation and X‐Ray Emission from Protostars

The ASCA satellite has recently detected variable hard X-ray emission from two class I protostars in the o Oph cloud, YLW 15 (IRS 43) and WL 6, with a characteristic timescale of D20 hr. In YLW 15, the X-ray emission is in the form of quasi-periodic energetic Nares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The Nare modeling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows us to calculate the rotation parameters of the star and the disk. In WL 6, X-ray Naring is rotationally modulated and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. On the basis of these observations, we Ðnd that YLW 15 is a fast rotator (near break-up), while WL 6 rotates with a signiÐcantly longer period. We thus use X-ray Naring as a clock II to measure the rota- tion of protostars. With the help of the mass-radius relation on the stellar birthline,II we derive masses of and for the central stars of YLW 15 and WL 6, respectively. YLW 15 thus M * D 2 M _ (0.4 M _ appears to be a future A star. In the long term, the magnetic interactions between the star and the disk result in magnetic braking and angular momentum loss of the star. A comparison of the rotation behav- ior of YLW 15 and WL 6 conÐrms that for solar-mass stars magnetic braking takes place on timescales few ) 105 yr, i.e., of the same order as the estimated duration of the class I protostar stage. The t br D a main parameter determining turns out to be the stellar mass, so that close to the birthline there must t br be a mass-rotation relation, such that stars with are fast rotators, while t br D P M * , M * Z 1E2 M _ their lower mass counterparts have had the time to spin down and reach synchronous rotation with the inner surrounding accretion disk. The rapid rotation and strong star-disk magnetic interactions of YLW 15 also naturally explain the observation of superNares II of X-ray luminosities as high as L X Z ergs s~1 during a few hours, while at the WL 6 stage the lower X-ray luminosities are likely 1033E1034 to be of purely stellar origin. The mass-rotation relation through magnetic braking may also explain why so few class I protostars have been detected in X-rays to date, and why they all lie in clusters. In the case of YLW 15, and perhaps also in other protostars, a hot coronal wind (T D 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the Ðrst clues to the magnetic properties of protostars, which govern their rotation status and evolution. Subject headings : stars : Nare E stars : individual (YLW 15, WL 6) E stars : magnetic Ðelds E stars : preEmain-sequence E stars : rotation E X-rays : stars

Star Formation in the Orion Nebula Cluster

We study the record of star formation activity within the dense cluster associated with the Orion Nebula. The bolometric luminosity function of 900 visible members is well matched by a simplified theoretical model for cluster formation. This model assumes that stars are produced at a constant rate and distributed according to the field-star initial mass function. Our best-fit age for the system, within this framework, is 2 ? 106?yr. To undertake a more detailed analysis, we present a new set of theoretical pre-main-sequence tracks. These cover all masses from 0.1 to 6.0?M?, and start from a realistic stellar birthline. The tracks end along a zero-age main-sequence that is in excellent agreement with the empirical one. As a further aid to cluster studies, we offer an heuristic procedure for the correction of pre-main-sequence luminosities and ages to account for the effects of unresolved binary companions. The Orion Nebula stars fall neatly between our birthline and zero-age main-sequence in the H-R diagram. All those more massive than about 8?M? lie close to the main sequence, as also predicted by theory. After accounting for the finite sensitivity of the underlying observations, we confirm that the population between 0.4 and 6.0?M? roughly follows a standard initial mass function. We see no evidence for a turnover at lower masses. We next use our tracks to compile stellar ages, also between 0.4 and 6.0?M?. Our age histogram reveals that star formation began at a low level some 107?yr ago and has gradually accelerated to the present epoch. The period of most active formation is indeed confined to a few ? 106?yr, and has recently ended with gas dispersal from the Trapezium. We argue that the acceleration in stellar births, which extends over a wide range in mass, reflects the gravitational contraction of the parent cloud spawning this cluster.

Direct Observations of the Ionizing Star in the Ultracompact HiiRegion G29.96−0.02: A Strong Constraint on the Stellar Birth Line for Massive Stars

We have observed the ultracompact H II region G29.96-0.02 in the near-infrared J, H, and K bands and in the Brγ line. By comparison with radio observations, we determine that the extinction to the nebula is AK = 2.14 with a 3 σ uncertainty of 0.25. We identify the ionizing star and determine its intrinsic K magnitude. The star does not have an infrared excess and so appears to be no longer accreting. The K magnitude and the bolometric luminosity allow us to place limits on the location of the ionizing star in the H-R diagram. The 3 σ upper limit on the effective temperature of the ionizing star is 42,500 K. We favor a luminosity appropriate for star with a mass in excess of about 60 M☉. The limit on the temperature and luminosity exclude stars on the zero-age main sequence (ZAMS) and stars within 106 yr of the ZAMS. Since the age of the UC H II region is estimated to be only about 105 yr, we suggest that this is direct evidence that the stellar birth line for massive stars at twice solar metallicity must be significantly redder than the ZAMS.

The fine structure of luminosity functions of young open cluster stars

Abstract The luminosity function (LF) of young open cluster stars has been studied for decades to obtain a slope of the initial mass function (IMF), which is important for understanding of star formation. Young open clusters contain a lot of pre-main-sequence (PMS) stars the positions of which in the H—R diagram deviate significantly from the main sequence and consequently the PMS massluminosity relation (MLR) differs from the ZAMS one. Such features of the MLR and related details of young open cluster LFs they produce are the subject of this poster. Three peaks have been studied: (1) the main peak (H-peak)—generated by PMS stars settling on the main sequence; (2) the R-peak—generated by inversion of the radius on the stellar birthline; the D-peak—generated by changes in the MLR and associated with deuterium burning. The evolution of these features with cluster age and possible application for young open cluster study are discussed.

Disk Accretion and the Stellar Birthline

We present a simplified analysis of some effects of disk accretion on the early evolution of fully convective, low-mass pre-main-sequence stars. Our analysis builds on the previous seminal work of Stahler, but it differs in that the accretion of material occurs over a small area of the stellar surface, such as through a disk or magnetospheric accretion column, so that most of the stellar photosphere is free to radiate to space. This boundary condition is similar to the limiting case considered by Palla & Stahler for intermediate-mass stars. We argue that for a wide variety of disk mass accretion rates, material will be added to the star with relatively small amounts of thermal energy. Protostellar evolution calculated assuming this limit of accretion generally follows the results of Stahler because of the thermostatic nature of deuterium fusion, which prevents protostars from contracting below a in the H-R diagram. Our calculated protostellar radii tend to fall below Stahler's at higher masses; the additional energy loss from the stellar photosphere in the case of disk accretion tends to make the protostar contract. The low-temperature disk accretion evolutionary tracks never fall below the deuterium-fusion birthline until the internal deuterium is depleted, but protostellar tracks can lie above the birthline in the H-R diagram if the initial radius of the protostellar core is large enough or if rapid disk accretion (such as might occur during FU Ori outbursts) adds significant amounts of thermal energy to the star. These possibilities cannot be ruled out by either theoretical arguments or observational constraints at present, so that individual protostars might evolve along a multiplicity of birthlines with a modest range of luminosity at a given mass. Our results indicate that there are large uncertainties in assigning ages for the youngest stars from H-R diagram positions, given the uncertainty in birthline positions. Our calculations also suggest that the relatively low disk accretion rates characteristic of T Tauri stars below the birthline cause low-mass stars to contract only slightly faster than normal Hayashi track evolution, so that ages for older pre-main-sequence stars estimated from H-R diagram positions are relatively secure.

Pre-main sequence candidates in the very young open cluster NGC 6611

For the search of Herbig Ae/Be objects in the extremely young open cluster NGC 6611 we have selected a sample of 52 pre-main sequence candidates, discovered by Walker (1961), Sagar & Joshi (1979), Chini & Wargau (1990) and The et al. (1990). We continue the approach of the last paper by studying each star individually with new and unpublished Walraven WULBV , Johnson/Cousins and Johnson JHKLM photometric data as well as low resolution spectroscopy. Each object is shown to have its own extinction law, which is investigated using their spectral energy distribution (SED). There does not seem to be a clear relationship between the location of a star and the extinction law. This means that the extinction is generated locally and its correction must be taken individually. For each object accurate astrophysical parameters are then derived. Plotting the objects in an HR-diagram, together with the values for the , the probability of membership value P and the extinction characteristics, helps to discriminate between cluster members and non-cluster members. Most foreground stars are of late spectral type and are labeled as Group III objects. Group I , to which most members of this cluster belong, contains objects of early spectral type. Part of them seem to be in their post-ZAMS phase and the other part in their pre-ZAMS stage. By comparing the evolutionary tracks of Palla & Stahler (1993) for pre-MS objects and of Maeder & Meyenet (1988) for post-MS stars we have concluded that the cluster contains objects of a few 0.1 Myr as well as objects of about 6 Myr. As most of the Group I objects do not show well-known Herbig Ae/Be characteristics, the time scale of clearing the disk material must be typically less than about 0.1 Myr for the more massive objects. Objects that show an IR-excess are found among the less luminous ones. They could still be in their pre-ZAMS phase, having an age of about 1 Myr. Such an age is appropriate for the Group II objects, which are of intermediate spectral type. As they are located close to the stellar birthline they should have been formed recently. It seems that an efficient clearing mechanism must have taken place, because not many of these objects are embedded and show only some IR-anomalies. This could be the reason that we have found no more than four typical Herbig Ae/Be candidates. We support the hypothesis of Hillenbrand et al. (1993) that there is an age spread in NGC 6611. The stars with the highest A V values are located in the centre of the cluster, somewhat to the northwest. This coincides with the location of many embedded sources more towards the northwest, a region in which star-formation is probably still taking place. The age of the most evolved objects is about 6 Myr. It would be interesting to study these regions and the Group II objects since investigating the youngest objects which are still partly embedded may help us to understand the first phases of star formation and the rapid cleaning of the circumstellar material. Young “naked” stars can also be formed by evaporating gaseous globules (EGGs) as was recently discovered in dark regions of this cluster (Hester & Scowen 1995). The lack of angular momentum of such objects could explain the paucity of HAeBe candidates.

Pre-Main-Sequence Evolution in the Taurus-Auriga Molecular Cloud

This paper analyzes optical and infrared photometry of pre-main-sequence stars in the Taurus-Auriga molecular cloud. More than half of the stars in our sample have excess near-infrared emission. The near-infrared excesses correlate very well with other measures of activity, such as Ho emission, ultraviolet excess emission, millimeter continuum emission, and the presence of reflection nebulae and molecular outflows. The infrared colors and the ratio of far-infrared to bolometric luminosity display a smooth progression from the most deeply embedded protostars to optically visible T Tauri stars. Infalling envelope models account for the colors of protostars; simple disk models similarly reproduce the colors of many T Tauri stars. <P />Both the stellar birth line and a l0<SUP>5</SUP> yr isochrone provide a reasonable upper envelope to the luminosity distribution of optically visible stars in the H-R diagram. Only a few stars in the cloud have apparent ages exceeding 2- 3 × 10<SUP>6</SUP> yr, as derived from detailed stellar evolution calculations. The distribution of stars in the H-R diagram indicates that the cloud has formed stars at a roughly constant rate for the past 1-2 × 10<SUP>6</SUP> yr. Analyses of the J and K-luminosity functions support this conclusion. Within the uncertainties, the observed mass distribution for optically visible stars agrees with a Miller-Scalo initial mass function. Source statistics imply a lifetime of 1-2 × l0<SUP>5</SUP> yr for the typical protostar in Taurus-Auriga. There is no evidence, however, that these sources lie on the stellar birth line. Indeed, the protostellar luminosity function is essentially identical to the luminosity function derived for optically visible T Tauri stars in the cloud. <P />These results provide some support for the evolutionary sequence-embedded protostar → T Tauri star with a circumstellar disk → T Tauri star without a circumstellar disk-currently envisioned in standard models of low-mass star formation. Source statistics and infrared color-color diagrams demonstrate that pre-main-sequence stars develop bluer colors and display less evidence for circumstellar material with time. The data show little evidence, however, for the luminosity evolution expected along the proposed evolutionary sequence. Time-dependent accretion during the infall phase may account for the low luminosity of Taurus-Auriga protostars; this hypothesis requires more tests.

The evolution of intermediate-mass protostars. II - Influence of the accretion flow

view Abstract Citations (150) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Intermediate-Mass Protostars. II. Influence of the Accretion Flow Palla, Francesco ; Stahler, Steven W. Abstract The growth of spherical protostars that condense from diffuse interstellar clouds is considered, with emphasis on the internal structure of protostars in the intermediate-mass range 2-10 solar masses. Three evolutionary sequences, one with a previous mass accretion rate of 0.00001 solar mass/yr, and the others with 0.00003 and 0.0001 solar mass/yr. The stellar birthline constructed with M-dot of 0.00001 solar mass/yr and either set of boundary conditions shows good agreement with the observed distribution of premain-sequence stars in the H-R diagram. It is proposed that all stars with less than about 10 solar masses form with an accretion rate of this order of magnitude, and from molecular cloud cores with similar density and temperature structures. It is speculated that high-mass stars, i.e., those with no visible premain-sequence phase, form with accretion rates no larger than about 0.0001 solar mass/yr. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171468 Bibcode: 1992ApJ...392..667P Keywords: Accretion Disks; Hertzsprung-Russell Diagram; Pre-Main Sequence Stars; Stellar Evolution; Stellar Interiors; Deuterium; Interstellar Matter; Molecular Clouds; Protostars; Stellar Mass; Astrophysics; ACCRETION; ACCRETION DISKS; STARS: HERTZSPRUNG-RUSSELL DIAGRAM; STARS: EVOLUTION; STARS: INTERIORS; STARS: PRE--MAIN-SEQUENCE full text sources ADS | Related Materials (1) Part 1: 1991ApJ...375..288P