Low-mass Pre-main Sequence Stars Research Articles

Young Stellar Populations around SN 1987A

We present the first results of a study of the stellar population in a region of 30 pc radius around SN 1987A, based on an analysis of multiband Hubble Space Telescope (HST) WFPC2 images. The effective temperature, radius and, possibly, reddening of each star were determined by fitting the measured broadband magnitudes to the ones calculated with model atmospheres. In particular, we have determined effective temperatures and bolometric luminosities for 21,995 stars, and for a subsample of 2510 stars we also determined individual reddening corrections. In addition, we have identified all stars with H? equivalent widths in excess of 8 ?, a total of 492 stars. An inspection of the H-R diagram reveals the presence of several generations of young stars, with ages between 1 and 150 Myr, superposed on a much older field population (0.6-6 Gyr). A substantial fraction of young stars with ages around 12 Myr make up the stellar generation coeval to SN 1987A progenitor. The youngest stars in the field appear to be strong-line T Tauri stars, identified on the basis of their conspicuous (Weq > 8 ?) H? excesses. This constitute the first positive detection of low-mass (about 1-2 M?) pre-main-sequence (PMS) stars outside the Milky Way. Their positions in the H-R diagram appear to require that star formation in the LMC occurs with accretion rates about 10 times higher than in the Milky Way, i.e., ~10-4 M? yr-1. SN 1987A appears to belong to a loose, young cluster 12 ? 2 Myr old, in which the slope of the present mass function is almost identical to Salpeter's, i.e., ? = d log N/d log M -1.25 for masses above 3 M?, but becomes much flatter for lower masses, i.e., ? -0.5. On a large scale, we find that the spatial distributions of massive stars and low-mass PMS stars are conclusively different, indicating that different star formation processes operate for high- and low-mass stars. This results casts doubts on the validity of an initial mass function (IMF) concept on a small scale (say, less than 10 pc). Moreover, it appears that a determination of the low-mass end IMF in the LMC requires an explicit identification of PMS stars. A preliminary analysis, done for the whole field as a single entity, shows that the IMF slope for the young population present over the entire region is steeper than ? -1.7.

ITAL]K[/ITAL]-Band Variability as a Method to Select Young Stellar Object Candidates

A study of the applicability of K- (2.2 km) band variability as a method to separate young stellar objects (YSOs) from —eld stars in young embedded clusters is presented. Deep K-band imaging of the central D8@ ) 6@ region of the Serpens Cloud Core was made at several epochs in 1995 and 1996. Varia- bility in K was detectable at peak-to-peak amplitudes as small as typically *K 0.15 at a conservative 3 p level for sources brighter than K B 15. This level increases gradually to *K 1.3 at K B 18. From a multi-epoch sample of about 1000 stars (K 18), a total of 55 stars have been found to exhibit variabil- ity in the K band at observed amplitudes from 0.15 to 2.2 mag, and nine of these have varied at several of the investigated epochs. Thirty-nine of the 55 variable sources were not previously recognized and are hereby proposed as new YSO candidates. Not less than 26% of the total sample of already known YSOs within this region were found to vary over a timescale of only 1 yr. We conclude that the K-variability method is efficient in —nding low-luminosity young sources in deeply embedded clusters. In addition, deep J (1.25 km) and H (1.65 km) imaging of the same region revealed 37 near-IR excess sources from their position in the J(H/H(K diagram, and 17 of these are new. One additional YSO candidate was found from association with nebulosity. The total of 57 new YSO candidates increases the currently rec- ognized YSO population by 85% within the observed —eld. The K luminosity function continues to rise down to K B 16, and no turnover is found at K ( 16 that might not be due to sample incompleteness. The new sources are probably very low mass premain-sequence stars and young brown dwarfs.

The History of Low-Mass Star Formation in the Upper Scorpius OB Association

We use a large sample of about 100 low-mass pre–main-sequence (PMS) stars in the Upper Scorpius OB association to explore the star formation history and the initial mass function of this association. Upper Scorpius is an ideal target for such a study, because the star formation process there is finished. The PMS stars have recently been found in a spatially unbiased wide-field survey of X-ray–selected stars in a 160 deg2 area, covering the Upper Scorpius association nearly completely. Following the optical characterization of these PMS stars, we present a new HR diagram for this association. We perform a detailed analysis of the HR diagram, taking proper account of the uncertainties and the effects of unresolved binaries, and derive ages and masses for the PMS stars. We find that the low-mass PMS stars have a mean age of about 5 Myr and show no evidence for a large age dispersion. This agrees very well with the age of 5–6 Myr previously found for the massive stars and shows that low-mass and high-mass stars are coeval and cospatial and thus have formed together. We conclude that the star formation process in Upper Scorpius was probably triggered by the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association. After a short burst of very high star formation activity, which lasted only for a few Myr, star formation in Upper Scorpius was halted, probably by the strong winds and the ionizing radiation of the numerous massive stars that dispersed the molecular cloud.

A Search for Very Low Mass Pre–Main-Sequence Stars in Taurus

?????We present the initial results of a deep CCD survey for very low mass pre?main-sequence (PMS) stars in selected fields of the Taurus molecular cloud complex. The results reported herein span a little over half a square degree in the dark clouds L1495, L1529, L1551, and B209. Our survey is complete down to IC ~ 19 at (R - I)C ~ 2.5, which enables us to probe well below the hydrogen burning limit at 1?2 Myr. From follow-up spectroscopic observations we have identified nine new low-mass T Tauri stars (TTSs). A large fraction of the new PMS objects (5/9) have very low masses as inferred from their late spectral types (?M5), and comparison with recent evolutionary tracks and Pleiades brown dwarfs suggests that our M6?M6.5 new TTSs are very young brown dwarfs. Two of the new TTSs may constitute a new, moderately embedded, binary classical T Tauri system. The new young stars represent a ~38% increase in the known PMS population of our survey area and a factor of ~2 increase in the number of known late-type TTSs. In spite of our sensitivity, we detect no young stars with spectral types later than ~M7. Our results illustrate the importance of spectroscopy in eliminating foreground M stars.

Chromospheric Activity of Weak-Lined T Tauri Stars

A summary of our ongoing high and medium resolution optical observational program, devoted to the study of chromospheric activity of pre-main sequence stars, is reported here. The study is centered in bona-fid weak-lined T Tauri stars (WTTS) in Taurus with known rotational period, and in WTTS recently discovered by the ROSAT All-Sky Survey. It is our purpose to quantify the phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric activity indicators (Ca II H & K, Hβ, Hα, Ca II IRT) as well as to obtain the Li I abundance, a better determination of the stellar parameters, spectral type, and possible binarity. By using these chromospheric excess emissions we will study the flux-flux and rotation-activity relationships in order to get insights into the mechanisms which drive solar-like stellar activity in this kind of low mass pre-main sequence stars.

Weak and Post–T Tauri Stars around B-Type Members of the Scorpius-Centaurus OB Association

I report medium-resolution (FWHM = 1.9 A) spectroscopic observations of six Hα emission-line stars proposed in 1993 by Meyer, Wilking, &Zinnecker to be T Tauri stars formed in the vicinity of the B1 giant σ Sco, a bright member of the Sco-Cen OB association. Using spectroscopic criteria (spectral types, Hα and Li I equivalent widths), which are distance-independent, I classify these stars in different pre–main-sequence (PMS) classes. Taking data from the literature, a number of stars detected by X-ray observations around other B-type members of Sco-Cen are also classified. The current census of bona fide low-mass PMS stars identified in about 9 deg2 in Sco-Cen includes two classical T Tauri stars, 18 weak T Tauri stars (WTTSs), and 10 post–T Tauri stars (PTTSs). The presence of a mixture of T Tauri and post–T Tauri stars implies that previous results based on isochrone fitting that indicated an extremely young age (~1 Myr) for the Sco-Cen PMS low-mass population are incorrect. A distance of about 125 pc for Sco-Cen, instead of the 160 pc used in previous works, is consistent with the Hipparcos parallaxes for many of the B-type stars and would lead to older H-R diagram ages. Taking into account that PTTSs are generally fainter and harder to identify than WTTSs, I argue that the WTTS-to-PTTS ratio in Sco-Cen may be of order unity. This result suggests that the low-mass stars of the OB association span an age range similar to the B-type members (5–15 Myr), i.e., the low- and high-mass star populations are essentially coeval. Sco-Cen appears to be indeed a promising place to find many PTTSs in future surveys.

Low-Mass Pre-Main Sequence Stars and Their X-ray Emission

To investigate the formation and early evolution of stars, astronomers study the x-ray emission of T Tauri stars, which are young, solar-mass stars called pre–main sequence stars. Two Earth-orbiting x-ray satellites, the Röntgen X-ray Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), have discovered x-ray emission from young protostars, called Class I objects. Many T Tauri stars were detected as x-ray sources by ROSAT. X-ray luminosity functions and correlations with other stellar parameters can be studied and used to investigate the x-ray emission mechanism. From the ROSAT data hundreds of T Tauri stars have been discovered, some of which are located outside regions of ongoing star formation. Stellar x-rays also irradiate circumstellar disks, regions where planets may form, so x-ray emission data from T Tauri stars may also be used to investigate the formation of planets.

Additional constraints on circumstellar disks in the Trapezium Cluster

We discuss the new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new Hubble Space Telescope (HST) Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than or approximately = 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although is remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25%-75% is comparable to that found for pre-main sequence (PMS) stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around PMS stars.

X-ray sources in regions of star formation. 5: The low mass stars of the Upper Scorpius association

We report followup investigations of Einstein x-ray observations of the Upper Scorpius OB association. We identify 28 low mass pre-main sequence stars as counterparts of x-ray sources in the approximately = 7 square degrees of the OB association observed. Based on kinematics and lithium abundances, these stars are low mass members of the OB association. We use optical spectra and optical and near-IR photometry to determine the stellar luminosities, effective temperatures, masses, and ages. We show that the bolometric corrections and effective temperatures of the G and K stars are consistent with those of subgiants. The low mass stars have isochronal ages of 1-2 Myr, depending on the choice of evolutionary models, with very small dispersion (sigma approximately = 1 Myr). This age is significantly younger than the 5-6 Myr found for the more massive B stars. The small dispersion in stellar ages, less than 10% the sound-crossing time of the association, suggests that star formation was triggered. We present two scenarios for star formation in this association. In the two-episode scenario, formation of the low mass stars was triggered by a supernova explosion, and the low mass stars form quickly, with high efficiency. Alternatively, high and low mass star formation was all initiated at the same time, some 5-6 Myr ago, and the apparent systematic age difference is an artifact of how the isochrones are dated. The effect of the supernova is to terminate mass accretion and yield an apparently coeval population. We show that the incompleteness in the x-ray sampling is about 65%, and is strongly dependent on stellar mass. After correction for incompleteness, we estimate there are about 2000 low mass members (stellar mass less than 2 solar mass) of this association. The mass function in this association is indistinguishable from that of the field. The ratio of naked to classical T Tauri stars is much larger than in Tau-Aur, and may be attributable to the local environment. We also present observations of eight ROX sources associated with the rho Oph cloud, and observations of non-pre-main sequence (PMS) stars in our fields.

The evidence for clumpy accretion in the Herbig Ae star HR 5999

Analysis of IUE high- and low-dispersion spectra of the young Herbig Ae star HR 5999 (HD 144668) covering 1978–1992 has revealed dramatic changes in the Mg II h and k (2795.5, 2802.7 A) emission profiles, changes in the column density and distribution in radial velocity of accreting gas, and flux in the Lyα, OI and CIV emission lines, which are correlated with the UV excess luminosity. We also observe variability in the spectral type inferred from the UV spectral energy distribution, ranging from A5 IV-III in high state to A7 III in the low state. The trend of earlier inferred spectral type with decreasing wavelength and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence stars (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M ≥ 2 M⊙) PMS star. Recent IUE spectra show gas accreting toward the star with velocities as high as +300 kms-1 much as is seen toward β Pic, and suggest that we also view this system through the debris disk. The absence of UV lines with the rotational broadening expected given the optical data (A7 IV, v sin i = 180 ± 20 km s-1) for this system also suggests that most of the UV light originates in the disk, even in the low continuum state. The dramatic variability in the column density of accreting gas, consistent with clumpy accretion, such as has been observed toward β Pic, is a hallmark of accretion onto young stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars with 0.5 < t age < 2.8 Myr. The implications for models of β Pic and similar systems are briefly discussed.

The Short-Period Binary Frequency Among Low-Mass Pre-Main Sequence Stars

The pre-main sequence (PMS) binary frequency is a fundamental datum in the study of binary formation. It reflects on numerous basic issues, such as:• The formation process. Binary stars are the primary branch of the star-formation process, and thus their frequency is an essential challenge to star-formation theories. (Indeed, the infrequency of single-star formation is likely as significant as the binary frequency.)• The epoch of binary formation. Assessing whether the binary population exists in total by the pre-main sequence phase sets an upper limit on the binary formation timescale.• Early period evolution. The frequency distribution as a function of period of PMS binaries, when compared to the distribution at the zero-age main sequence, can shed light on early orbital evolution.• The interaction of binaries with disks. The formation and consequent dynamical evolution of a binary with semi-major axis less than typical disk radii must substantially modify disk structures and accretion flows. Thus the binary frequency might differ between PMS stars with and without associated disks.

Magnetic Activity of T Tauri Stars

T Tauri stars (TTS) are low-mass (M ≲ 1M⊙) pre-main sequence (PMS) stars (for a general review, see Bertout 1989). They have long been known to be variable from near-TIV to near-IR wavelengths, on timescales ranging from a few minutes to a few decades. They are observed to flare in many wavenlength rages, from X-rays to the radio, and all the existing evidence is consistent with a very strong magnetic activity, in many ways analogous to solar activity (for a review, see, e.g., Montmerle et al. 1991).

Spectroscopic Binaries Among Low-Mass Pre-Main Sequence Stars

Although the unusual nature of T Tau was noted over four decades ago, the first orbit for a low-mass pre-main sequence (PMS) spectroscopic binary was not determined until Mundt et al. (1983) serendipitously discovered the double-lined nature of V826 Tau. To some degree the paucity of spectroscopic binary detections may be attributed to the faintness of such stars and the consequent difficulty in obtaining high resolution spectra; the first high-precision radial-velocity survey was that of Herbig (1977). Advances in radial-velocity measurement technology now permit relatively easy velocity measurements of PMS stars. However, V826 Tau was also one of the first discovered members of the naked T Tauri (NTTS) class of PMS stars (Walter 1987) and its discovery as a spectroscopic binary foreshadowed a prevalence for binary detection among this population. In this short paper we review the present observational status of PMS spectroscopic binaries and present several initial results and thoughts for consideration.

THE NAKED T TAURI STARS : THE LOW-MASS PRE-MAIN SEQUENCE UNVEILED.

ABSTRACT The author discusses the search for low-mass pre-main-sequence (PMS) stars associated with X-ray sources in regions of star formation. The survey to date has revealed at least 30 low-mass PMS stars in the Taurus-Auriga region, and a comparable number in Ophiuchus. These stars are the naked T Tauri stars, unveiled versions of the well-known classical T Tauri stars. The author discusses the properties of these newly discovered PMS stars and their relation to the classical T Tauri stars. He concludes that the naked T Tauri stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tauri stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. He discusses the impact the naked T Tauri stars are likely to have on our understanding of the PMS evolution of low-mass stars.

Far-infrared photometry of low-mass pre-main-sequence stars with broad CO wings

A sample of 14 low-mass premain-sequence stars were studied in the far-infrared. There is evidence for outflowing material around twelve of these stars; in most cases the evidence includes broad wings on the profiles of the CO J = 2 to 1 line. Ten of the objects were detected (5 sigma) at 100 micros, and seven were mapped at 50 and 100 micros. The far-infrared emission makes a significant contribution to the total luminosity of the objects. It is found that the force available from radiation pressure is not sufficient to drive the outflows. Any process which taps the same energy source as does the present stellar luminosity must have high efficiency.

JHKL photometry of three Herbig-Haro jet sources and a nebulous T Tauri star

Infrared sources have been detected at the positions of the Herbig-Haro jet sources HH 30, DG Tau B, and Haro 6-5 B in the Taurus dark cloud complex. The colors of these Herbig-Haro jet sources in the 1-4 micron wavelength range are comparable to those of the dustiest T Tauri stars, while the K magnitudes corrected for likely interstellar extinction are actually somewhat fainter than those oftypical T Tauri stars in this region. These observations are consistent with the view that these Herbig-Haro jet sources are low-mass premain-sequence stars. The nebulous T Tauri star Haro 6-5 was also observed and found to show both a large infrared excess and an intrinsic-polarization of about 3 percent in visible light. Thus this star seems to resemble HL Tau and DG Tau, which have been suggested as examples of young stars with circumstellar disks. 17 references.

A Note about Multiple Systems of Dense Molecular Clouds

AbstractThe space distribution of some small dense clouds with point-like IR sources, resembles the clustering of young OB stars. It can be assumed that such objects contain heavy obscured high-luminosity stars on the ZAMS. From the comparison of infrared and radio data it follows that in typical cases, only one B star is the source of the radiation of the cloud. The total mass of the cloud is of the order of one solar mass. If the individual fragments of the cloud are gravitationally unstable, then in the later stage of the evolution only low-mass stars are formed. One can therefore expect that young OB stars are most frequently accompanied by low-mass pre-Main Sequence stars.