Showing papers on "Star formation published in 1993"

Submillimeter Continuum Observations of rho Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps

Abstract: We have mapped the submillimeter continuum emission from the rich star-forming core, ρ Oph A, at 350, 450, 800, and 1300 μm using the JCMT and IRAM 30 m telescopes. In addition to the diffuse emission from the cloud core itself, these observations reveal four well-defined clumps, which, suprisingly, are barely visible in the single-dish molecular maps obtained so far in this region. The gas may have partially frozen out onto the dust grains, due to low temperature and/or high density in the clumps. We discuss the possibility that these clumps are prestellar or extremely young, low-mass protostars

The multiplicity of T Tauri stars in the star forming regions Taurus-Auriga and Ophiuchus-Scorpius: A 2.2 micron speckle imaging survey

Abstract: We present the results of a magnitude limited (K less than = 8.5 mag) speckle imaging survey of 69 T Tauri stars in the star forming regions Taurus-Auriga and Ophiuchus-Scorpius. Thirty-three companion stars were found with separations ranging from 0.07 sec to 2.5 sec, nine are new detections. This survey reveals a distinction between the classical T Tauri stars (CTTS) and the weak-lined T Tauri stars (WTTS) based on the binary star frequency as a function of separation: the WTTS binary star distribution is enhanced at the closer separations (less than 50 AU) relative to the CTTS binary star distribution. We suggest that the nearby companion stars shorten the accretion time scale in multiple star systems, thereby accounting for the presence of WTTS that are coeval with many CTTS. The binary star frequency in the projected linear separation range 16 to 252 AU for T Tauri stars (60 (+/- 17)%) is a factor of 4 greater than that of the solar-type main-sequence stars (16(+/- 3)%). Given the limited separation range of this survey, the rate at which binaries are detected suggests that most, if not all, T Tauri stars have companions. We propose that the observed overabundance of companions of T Tauri stars is an evolutionary effect, in which triple and higher order T Tauri stars are disrupted by close encounters with another star or system of stars.

Massive star formation near the Galactic center and the fate of the stellar remnants

Abstract: Several points are made regarding massive stars, star formation, and stellar remnants in the Galactic center region, particularly the inner 1-10 parsecs. First, in light of the processes which act to inhibit or suppress star formation there, it is argued that those stars which do form tend to be formed by an externally caused compression of their parent clouds rather than by spontaneous cloud collapse. As a result of this, and because of the particular characteristics of the interstellar medium near the Galactic center, it is likely that the initial mass function (IMF) favors more massive stars than that in the Galactic disk, or at least that the lower mass cutoff of the Galactic center IMF is relatively large

The Chemical Evolution of QSOs and the Implications for Cosmology and Galaxy Formation

Abstract: We examine the chemical evolution of QSO broad-line gas by applying spectral synthesis and chemical enrichment models to the N V/C IV and N V/He II emission-line ratios. The models indicate that BLR metallicities are typically ∼1 to perhaps ≥10 times solar. The enrichment must occur in ≤1 Gyr for sources where the redshift is ≥3 (if q 0 =1/2). The higher metallicity QSOs require star formation favoring massive stars (compared to the Galactic disk). These results imply that extensive evolution usually occurs before the QSOs become observable. Our models of the evolution are equivalent to models proposed for elliptical galaxies and for the bulges of disk galaxies. We conclude that the QSO phenomenon is preceded by vigorous star formation, exactly like that expected in massive, young galactic nuclei

Lyman-Alpha Emission from Galaxies

Abstract: We use new models of stellar population synthesis to compute the Lyα emission from galaxies with different star formation histories and initial mass functions. The models include all phases of stellar evolution and recent advances in the theories of stellar interiors and atmospheres. We find that dust-free galaxies would have Lyα equivalent widths of 50-200 A, i.e., significantly higher than previous estimates, except from a few times 10 7 to 10 9 yr after a burst of star formation. We also consider several other factors that can affect the observed Lyα emission: the contributions by supernova remnants and active galactic nuclei, the orientation of a galaxy, and absorption by dust

Evidence for Protostellar Collapse in B335

Abstract: We have observed five rotational transitions of H2CO and CS toward the Bok globule, B335, with high spatial and spectral resolution. The characteristic shape of the observed profiles provides direct, kinematic evidence of collapse. In addition, we have modeled line profiles of collapsing dense cores with density and velocity structures taken from the theory of Shu and coworkers. Using the age of collapse as the only free parameter, we found that the strengths and profiles of the observed lines can be well fitted by the theoretical model. Our best-fit model gives an age of 1.5 x 10 exp 5 yr, corresponding to an infall radius of 0.04 pc and a total mass of 0.4 solar mass for the central star and disk. Outside the infall radius, there is a static envelope with a r exp -2 density distribution, an average temperature of 13 K, and a turbulent velocity (1/e width) of 0.12 km/s. The CS abundance is 3.6 x 10 exp -9 with about 30 percent uncertainty.

Ambipolar diffusion and star formation : formation and contraction of axisymmetric cloud cores. II: Results

Abstract: The problem of the formation and contraction of fragments (or cores) in magnetically supported parent molecular clouds was formulated in a previous paper. Three dimensionless free parameters appear in the evolution equations: the initial ratio of the free-fall and neutral-ion collision times (in the uniform reference state), v ff,0 the exponent k in the relation between the ion and neutral densities n i ∞n n k , and the initial ratio of the magnetic and thermal pressures, α 0 . The initial central mass-to-flux ratio in units of the critical value for gravitational collapse, α 0 , enters through the initial conditions. We follow both the quasistatic and dynamicphases of contraction and demonstrate that ambipolar diffusion leads to self-initiated protostar formation ( quasistatic meaning motion with negligible acceleration)

On the spatial distribution of pre-main-sequence stars in Taurus

Abstract: We derive characteristic properties of the nonrandom spatial distribution of pre-main-sequence stars in the Taurus-Auriga molecular cloud by applying several different statistical techniques. We find that a power-law form for the two-point angular correlation function (with index −1.2) reproduces the overall shape of the actual pre-main-sequence distribution in Taurus at small angular scales (∼0.3°). This result is consistent with the existence of real clustering in the T Tauri distribution. With the aid of the nearest-neighbor distribution technique, we determine a median projected separation of −0.3 pc for young stars in this cloud, even after eliminating close pairs with separations less than 20″ (−3000 AU at the distance of Taurus) from our sample

Chemical differentiation between star-forming regions : the Orion Hot Core and Compact Ridge

Abstract: We present a dynamical-chemical model of massive star-forming regions, in which gas and dust grains are included. We consider the last 10 5 . yr of the accretion phase of a protostellar object embedded in a dense and massive cloud with a density and temperature gradient. We follow the gas and grain chemical evolution of two collapsing shells of this cloud, until the end of the protostar accretion phase, at which time the density no longer increases. At this point, the temperature rises, the molecular mantles of the grains evaporate, and we follow the time evolution of the resultant gas chemistry. The scenario is based on earlier models of Millar, Brown, Charnley, and Tielens

Explosive ejection of matter associated with star formation in the Orion nebula

Abstract: TIGHTLY collimated outflows of gas are often associated with regions of star formation; they interact with the ambient interstellar medium to produce the knots of shock-excited emission known as Herbig–Haro (HH) objects1,2. Two interpretations have been suggested for these objects: they may represent either the shocking of dense clumps of material that have been ejected into the surrounding molecular cloud3, or the interaction of stationary knots with fast, low-density jets4,5. Here we report the discovery of a complex of HH objects and associated wakes that require compact knots of material to have been ejected over a wide opening angle in a seemingly explosive event. Our observations suggest that, at least in this case, the former interpretation is correct, and they highlight the need to search other star-forming regions in order to establish the frequency of such events.

The H to H2 transition in galaxies - Totally molecular galaxies

Abstract: The transition from H to H 2 in the general interstellar medium is shown to depend sensitively on the pressure and radiation field, so even a slight increase in the ambient pressure can convert a whole cloud population into H 2 molecules without necessarily converting the diffuse clouds into self-gravitating clouds. Similarly, a slight increase in radiation can convert the clouds back to H. The calculation includes self-shielding and mutual shielding of line radiation between clouds, as well as dust extinction for diffuse and self-gravitating clouds with an M -1.5 mass distribution

Starburst Modeling of M82: Test Case for a Biased Initial Mass Function

Abstract: We compute starburst models for M82, making use of recent theoretical tracks of stellar evolution. Detailed comparisons of our models and those of others demonstrate this technique to be quite reliable, with relatively little change in output parameters as a function of the selection of theoretical tracks or of estimates of the observational characteristics of the stars along these tracks. The models are matched to the observational constraints for M82 summarized by McLeod et al. (1993). The rate of star formation and time of observation were thoroughly optimized to produce the most favorable fit to the observations, but we still found that the recently proposed forms for the solar-neighborhood IMF cannot produce starbursts adequate to fit the observations of this galaxy. We then explored adjustments to the shape of the IMF to improve the fit to M82. We find (1) the shape of the IMF for high-mass stars need not be different from that observed locally; and (2) the most likely modification to the IMF in M82 is that stars with masses below a few solar masses form much less commonly than in the solar neighborhood.

The embedded young stars in the Taurus-Auriga molecular cloud. I - Models for spectral energy distributions

Abstract: We describe radiative transfer calculations of infalling, dusty envelopes surrounding pre-main-sequence stars and use these models to derive physical properties for a sample of 21 heavily reddened young stars in the Taurus-Auriga molecular cloud. The density distributions needed to match the FIR peaks in the spectral energy distributions of these embedded sources suggest mass infall rates similar to those predicted for simple thermally supported clouds with temperatures about 10 K. Unless the dust opacities are badly in error, our models require substantial departures from spherical symmetry in the envelopes of all sources. These flattened envelopes may be produced by a combination of rotation and cavities excavated by bipolar flows. The rotating infall models of Terebey et al. (1984) models indicate a centrifugal radius of about 70 AU for many objects if rotation is the only important physical effect, and this radius is reasonably consistent with typical estimates for the sizes of circumstellar disks around T Tauri stars.

Formaldehyde as a probe of physical conditions in dense molecular clouds

Abstract: We present a detailed analysis describing the utility of the formaldehyde (H 2 CO) molecule in the derivation of the kinetic temperature and spatial density within molecular clouds. Measurements of 13 transitions from both the ortho and para species of H 2 CO have been made toward a sample of 11 active star formation regions. These H 2 CO transitions range in frequency from 211 to 365 GHz and in upper-state energy from 21 to 241 K. This range in excitation has allowed us to analyze H 2 CO sensitivity to both cool (T K ≤ 50 K) and warm (T K > 50 K) molecular material. Using a spherical large velocity gradient (LVG) model to solve for the excitation of H 2 CO, we analyze the sensitivity of several ortho- and para-H 2 CO transition intensity ratios to the kinetic temperature and spatial density within molecular clouds

Molecular Cloud Cores in the Orion A Cloud. I. Nobeyama CS (1--0) Survey

Abstract: A first high-resolution survey of molecular cloud cores in the Orion A giant molecular cloud is reported. We identified 125 molecular cloud cores from an analysis of the spatial and velocity distribution of the CS (1-0) emission. The cores are generally elongated along the filamentary molecular cloud, and the axial ratio is about 0.5. The mass spectrum index of the cores is -1.6 for M≥50 M ○ .. The physical properties of the cores identified in Orion are compared with those of cores in dark clouds reported in the literature. The average radius of the cores in the Orion A cloud, 0.16 pc, is comparable to that of the cores in dark clouds

Dust and chemistry in astronomy

Abstract: DUST AND ASTROCHEMISTRY: AN INTRODUCTION by T.J. Millar and D.A. Williams The Past. The Present. The Future. The Astronomical Background. Further Reading. OBSERVATIONS OF MOLECULAR ICES by D.C.B. Whittet Infrared Spectroscopy. The Observed Features. Conclusions and Future Research. OBSERVATIONS OF HOT MOLECULAR CORES by C.M. Walmsley and P. Schilke Observations of the Orion Hot Core. HCN Isotopomers Toward the Orion Hot Core. Observations of Other Hot Cores. Determining Physical Parameters of Hot Cores. Prospects. MOLECULAR HYDROGEN EMISSION FROM STAR-FORMING REGIONS by P.W.J.L. Brand Star-Forming Regions. Shocks. Photodissociation Regions. CARBONACEOUS GRAINS by W.W. Duley Bonding in Carbon Compounds. Structure of Solids. Formation of Carbon Solids. Constraints on Bonding in HAC. Electronic Transitions. Luminescence. Infrared Spectra. Comparison with Astronomical Spectra. What Is Carbonaceous Dust. Conclusions. PAHS AND ASTROCHEMISTRY by A.G.G.M. Tielens The IR Emission Features. PAHs and C-Stardust Formation. Photochemistry of PAHs. PAHs and Ion-Molecule Chemistry. Surface Chemistry of PAHs. PHYSICAL AND CHEMICAL PROCESSES ON DUST by D.A. Williams The Nature of Interstellar Dust and its Interaction with Gaseous Species. Sticking Probabilities. Mobility of Adsorbed Species. Reactions between Adsorbed Species. Ejection or Retention of Reaction Products. Mantle Processing. Desorption Processes on Interstellar Dust. Conclusion. INTERSTELLAR DUST AND THE CHEMISTRY OF DIFFUSE CLOUDS by R. Wagenblast and D.A. Williams The Role of Dust in Diffuse Cloud Chemistry. HO2 (IJ) Formation on Dust. Chemical Modeling of Diffuse Clouds: Hydrides from Dust. Hydrocarbons from Shocked Dust. GAS-GRAIN CHEMISTRY IN QUIESCENT DENSE INTERSTELLAR CLOUDS by E. Herbst Why Grain Chemistry Is Needed. Previous Models with Grain Reactions. Surface Processes: More Detail. A New Gas-Grain Chemical Model. Future Work. THE CHEMISTRY IN LOW MASS STAR FORMATION by T.W. Hartquist et al. Clusters of Star-Forming Cores. The Initial State Clumps. The Collapse of Initial State Clumps. Chemistry in a Core Cluster with Low Mass Stars. Core Collapse. Relating the Core Cluster and Collapsing Core Chemistries to the Composition of Comet Halley's Ices. A Wish List of Observations. CLUMP COLLAPSE AND STAR FORMATION by J.M.C. Rawlings The Environment and Identification of Low Mass Star-Forming Regions. Observations of Dense Cores. A Dynamical Model of the Chemistry in Star-Forming Regions. Concluding Remarks. THE CHEMISTRY OF HOT MOLECULAR CORES by T.J. Millar General Considerations. Models of Orion Sources. Chemical Differentiation in the Orion Hot Cores. Other Issues. Conclusions. LABORATORY DUST STUDIES AND GAS-GRAIN CHEMISTRY by J.M. Greenberg et al. Basic Experimental Set-Up. Some Examples of Photochemical Evolution in Dust. Molecule Desorption. Dust and Gas in Diffuse (Translucent) and Dense Clouds. PAH Survival and Destruction. Concluding Remarks. IRRADIATION OF MOLECULAR ICES by V. Pirronello Energy Deposition by Particles in Matter. Chemical Effects. Mechanisms of Chemical Effects. Erosion. Applications to Cosmic Dust.

Star Formation at Compressed Interfaces in Turbulent Self-gravitating Clouds

Abstract: Supersonic turbulence in both self-gravitating and diffuse clouds can produce self-similar, hierarchical density structures and broad line-wings on a wide range of scales, making these two cloud types look similar and giving the clumps in each the same scaling laws. We show here that the dense compressed regions that form between converging turbulent flows should collapse to form dense cores and ultimately stars in self-gravitating clouds, whereas they only disperse and reform intermittently in diffuse clouds. Star formation in turbulence-compressed regions should be as hierarchical as the turbulence, leading to clusters and multiple star systems instead of single stars; some binary stars may form by gravitational capture of stars in nearby turbulent clumps

Modeling the Spatial Distribution of Star Formation in Interacting Disk Galaxies

Abstract: Models of star-forming interacting galaxies are combined with observational data to investigate the nature of star formation in interacting and merging disk galaxies. Detailed models of specific interacting systems are created using the observed morphology and kinematics of the system to constrain model parameters describing the interaction. The models employ an N-body code to calculate the gravitational dynamics, a discrete cloud model to govern the ISM dynamics, and a modified Schmidt law to describe star formation

A survey for extra-H II region ionized gas structures in irregular galaxies

Abstract: We have conducted a deep Hα imaging survey of 51 irregular and amorphous galaxies with the objective of searching for large ionized gas structures outside of normal H II regions. In this sample 12% of the galaxies contain at least one ionized supershell (radius≥3OO pc), 24% contain supergiant ionized filaments which are not obviously connected with a particular star forming region, and 27% contain one or more of these types of structures. For the most part, large ionized gas structures are found in galaxies that are engaging in intense star formation or that contain at least one unusually large concentration of massive stars. Thus, in most galaxies these structures are likely to have been produced by massive stars

Star Trapping and Metallicity Enrichment in Quasars and Active Galactic Nuclei

Abstract: Recent observational evidence suggests that the metallicity in quasars within a wide range of redshifts, in particular in gas flowing out of the nuclear regions, may be approximately redshift-independent and comparable with or larger than solar. It is plausible that the nuclear metallicity can be internally generated and maintained at approximately time-stationary values in quasars. We identify and estimate efficiency of a mechanism for rapid metallicity enrichment of quasar nuclear gas (in general, in active galactic nuclei) based on star-gas interactions and equivalent to an unusual mode of massive star formation. The mechanism involves capture of low-mass stars from the host galaxy's nucleus by the assemblages of clouds or by accretion disks orbiting the central massive objects (e.g., black holes). Trapping of stars within gaseous disks/clouds occurs through resonant density and bending wave excitation, as well as by hydrodynamical drag. The time scale for trapping stars with total mass equal to that of disk fragment/cloud is of order Hubble time and is remarkably model-independent. Our results show that the described mechanism can produce features suggested by observations, for example, the (super) solar gas metallicity in the nucleus. Thus the observed metallicities in high-redshift quasars do not necessarily imply that global star formation and efficient chemical changes have occurred in their host galaxies at very early cosmological epochs.

Ambipolar Diffusion, Interstellar Dust, and the Formation of Cloud Cores and Protostars. I. Basic Physics and Formulation of the Problem

Abstract: We formulate the problem of the self-initiated formation and contraction of cloud cores due to ambipolar diffusion in axisymmetric, self-gravitating, isothermal, magnetic molecular cloud models, accounting for a cosmic abundance of interstellar grains (both charged and neutral). The basic microscopic and macroscopic effects of grains are discussed. The magnetohydrodynamic equations for a multicomponent plasma are derived, the dimensionless free parameters are obtained, and their meaning is explained physically. The ion and electron fluids combine to form a system of quasiparticles, each having an effective charge Z eff =e(n i -n e )/n i (where n i and n e are the number densities of ions and electrons, and e the electronic charge)

Are wide pre-main-sequence binaries coeval?

Abstract: We have observed a sample of 39 wide (projected separations 400-6000 AU) pre-main-sequence binary pairs spectroscopically and with optical and near-infrared images. The observations enable us to place 26 of the pairs in an H-R diagram and to determine masses and ages of the primary and secondary according to three sets of pre-main-sequence evolutionary tracks. In two-thirds of the cases the primary and secondary lie along the same isochrone to within the observational errors. However, real age differences appear for about one-third of our sample pairs-there is no set of nonintersecting theoretical isochrones that can make the primary and secondary have the same age for all pairs in our sample. In the cases where there are significant age differences between the component stars, the less massive star is usually younger than the more massive star. There is no correlation of the age differences with the presence or absence of accretion disks around the young stars. Hence, while disk accretion may affect the evolutionary tracks of the pre-main-sequence stars H-R diagram, we see no clear evidence of this effect among the pairs in our sample. The age differences also do not depend systematically on the apparent separation, the mass ratio, or the ages of stars.

Dynamical instabilities in two-phase media and the minimum masses of stellar systems

Abstract: Two-phase media, with cool dense clouds in pressure equilibrium with a hot, tenuous background from which they have cooled, have a prominent place in astrophysics, possibly being involved in the evolution of cooling flows, active galactic nuclei, and the formation of galaxies, star clusters and individual stars. Following their formation, the cool clouds are subject to Kelvin-Helmholtz instability, which grows as they move through the background gas. The instability is suppressed if the clouds are bound by a sufficiently strong gravitational potential. Analytical estimates of the growth and stability requirements of clouds are presented and are compared with the results of two-dimensional hydrodynamical simulations

Molecular line emission from circumstellar disks

Abstract: Calculations of the molecular line emission from circumstellar disks around young stars show the transitions of CO and its common isotopes 13 CO and C 18 O to have typical line-center optical depths between 100 and 1000. The line emission reflects the temperature distribution within the disks and not the mass distribution as previously suggested. The 13 CO line fluxes from the HL Tau and two similar stars show that the outer disk temperatures are, indeed, higher than expected for normal accretion disks, as inferred from far-infrared continuum data

The Embedded Young Stars in the Taurus-Auriga Molecular Cloud. II. Models for Scattered Light Images

Abstract: We describe NIR imaging observations of embedded young stars in the Taurus-Auriga molecular cloud. We find a large range in J-K and H-K colors for these class I sources. The bluest objects have colors similar to the reddest T Tauri stars in the cloud; redder objects lie slightly above the reddening line for standard ISM dust and have apparent K extinctions of up to 5 mag. Most of these sources also show extended NIR emission on scales of 10-20 arcsec which corresponds to linear sizes of 1500-3000 AU. The NIR colors and nebular morphologies for this sample and the magnitude of linear polarization in several sources suggest scattered light produces most of the NIR emission in these objects. We present modeling results that suggest mass infall rates that agree with predictions for cold clouds and are generally consistent with rates estimated from radiative equilibrium models. For reasonable dust grain parameters, the range of colors and extinctions require flattened density distributions with polar cavities evacuated by bipolar outflows. These results support the idea that infall and outflow occur simultaneously in deeply embedded bipolar outflow sources. The data also indicate fairly large centrifugal radii and large inclinations to the rotational axis for a typical source.

Structural Characteristics and Stellar Composition of Low Surface Brightness Disk Galaxies

Abstract: We present UBVI surface photometry of a sample of low surface brightness (LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential disks with no preferred value for either scale length, central surface brightness, or rotational velocity. Indeed, the distribution of scale lengths is indistinguishable from that of high surface brightness spirals, indicating that dynamically similar galaxies (e.g., those with comparable Rv^2) exist over a large range in surface density. These LSB galaxies are strikingly blue. The complete lack of correlation between central surface brightness and color rules out any fading scenario. Similarly, the oxygen abundances inferred from HII region spectra are uncorrelated with color so the low metallicities are not the primary cause of the blue colors. While these are difficult to interpret in the absence of significant star formation, the most plausible scenario is a stellar population with a young mean age stemming from late formation and subsequent slow evolution. These properties suggest that LSB disks formed from low initial overdensities with correspondingly late collapse times.

The genesis of starbursts and infrared emission in the centers of galaxies

Abstract: We present 10.8-micron maps of the central regions of 21 IR-luminous galaxies. Each map, obtained using the NASA MSFC bolometer array at the IRTF, has a resolution of about 4 arcsec and spans typically 25 x 20 arcsec. Some of the galaxies have also been mapped at 12.5, 19.2, and 30/microns. The sample consists primarily of starburst galaxies but includes several Seyferts and LINERs. On average, the flux detected in each map is about 58 percent of that detected at 12 microns by IRAS. We see a variety of morphologies, including compact and elongated sources and more complex distributions exhibiting such features as kiloparsec-size rings and double lobes. The mid-IR radiation is emitted by dust particles, and for two of the galaxies we see spatial color variations attributable to the presence of very small grains and consistent with their depletion in the starbursts. The mid-IR emission traces the dust heated by the youngest stars. Comparison of the distributions of intense star formation and the dense neutral interstellar medium for eight of these galaxies demonstrates that the two are intimately linked.

A Multiwavelength Study of Star Formation in the L1495E Cloud in Taurus

Abstract: We have carried out a deep (t=30000s) x-ray search of the eastern portion of the L1495 cloud centered on the well known weak line T Tauri star (WTTS) V410 Tau using the ROSAT PSPC. This deep exposure enabled a search for candidate pre-main sequence (PMS) objects in this cloud to a limit 20 times more sensitive than that typical of the fields examined with the Einstein searches. Despite assertions that the PMS population in Taurus-Auriga is nearly completely known, this x-ray survey revealed 8 new PMS objects in a region 50' in diameter, as compared to a previously known stellar population of 12 objects, including deeply embedded IRAS sources. Spectroscopic and photometric observations enable us to place these objects in the HR Diagram. The newly discovered objects are predominantly stars of spectral type M0 and later, and a large fraction (6/8) appear to be surrounded by circumstellar accretion disks as judged by their infrared excess and H alpha emission. We combined the data for these x-ray discovered objects with extant and new data for the previously identified PMS stars in this region to examine the history of star formation and the frequency distribution of stellar masses in this cloud. If the ``post ROSAT'' population is either complete or representative, we conclude (1) that star formation in L1495 East took place 1 x 10^6 yrs ago and that the spread in ages is small; (2) the frequency distribution of masses,