A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions would be the Rosetta Stone for star formation.



In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter.



Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Mendez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.

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On the variation of the initial mass function

We report on our search for microlensing toward the Large Magellanic Cloud (LMC). Analysis of 5.7 yr of photometry on 11.9 million stars in the LMC reveals 13-17 microlensing events. A detailed treatment of our detection efficiency shows that this is significantly more than the ~2-4 events expected from lensing by known stellar populations. The timescales () of the events range from 34 to 230 days. We estimate the microlensing optical depth toward the LMC from events with 2 < < 400 days to be τ = 1.2 × 10-7, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum-likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8%-50%. A 100% MACHO halo is ruled out at the 95% confidence level for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between 0.15 and 0.9 M☉, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 × 1010 M☉, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of 2. This is mostly due to Poisson noise, because with 3.4 times more exposure and increased sensitivity to long-timescale events, we did not find the expected factor of ~4 more events. In addition to a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing. We note that an important source of background are supernovae (SNe) in galaxies behind the LMC.

https://iopscience.iop.org/article/10.1086/309512/pdf

The MACHO Project: Microlensing Results from 5.7 Years of Large Magellanic Cloud Observations

▪ Abstract Observational studies of low-mass stars during their early stages of evolution, from protostars through the zero-age main sequence, show highly elevated levels of magnetic activity. This activity includes strong fields covering much of the stellar surface and powerful magnetic reconnection flares seen in the X-ray and radio bands. The flaring may occur in the stellar magnetosphere, at the star-disk interface, or above the circumstellar disk. Ionization from the resulting high-energy radiation may have important effects on the astrophysics of the disk, such as promotion of accretion and coupling to outflows, and on the surrounding interstellar medium. The bombardment of solids in the solar nebula by flare shocks and energetic particles may account for various properties of meteorites, such as chondrule melting and spallogenic isotopes. X-ray surveys also improve our samples of young stars, particularly in the weak-lined T Tauri phase after disks have dissipated, with implications for our underst...

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High-Energy Processes in Young Stellar Objects

In the light of recent significant progress on both the observational and theoretical fronts, we review the status of white dwarf stars as cosmochronometers. These objects represent the end products of stellar evolution for the vast majority of stars and, as such, can be used to constrain the ages of various populations of evolved stars in the Galaxy. For example, the oldest white dwarfs in the solar neighborhood (the remnants of the very first generation of intermediate-mass stars in the Galactic disk) are still visible and can be used, in conjunction with cooling theory, to estimate the age of the disk. More recent observations suggest the tantalizing possibility that a population of very old white dwarfs inhabits the Galactic halo. Such a population may contribute significantly to baryonic "dark" matter in the Milky Way and may be used to obtain an independent estimate of the age of the halo. In addition, white dwarf cosmochronology is likely to play a very significant role in the coming era of giant 8-10 m telescopes when faint white dwarf populations should be routinely discovered and studied in open and globular clusters.

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The Potential of White Dwarf Cosmochronology

In this, the first in a series of three papers concerning the SuperCOSMOS Sky Survey (SSS), we give an introduction and user guide to the survey programme. We briefly describe other wide-field surveys and compare them with our own. We give examples of the data, and make a comparison of the accuracies of the various image parameters available with those from the other surveys providing similar data; we show that the SSS data base and interface offer advantages over these surveys. Some science applications of the data are also described and some limitations discussed. The series of three papers constitutes a comprehensive description and user guide for the SSS.

The SuperCOSMOS Sky Survey – I. Introduction and description

Two soft X-ray images of the Chamaeleon I star-forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and 19 possible additional, X-ray sources are found. Eighty percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Nineteen to 39 are proposed new weak T Tauri (WTT) stars which, when confirmed by optical spectroscopy, will significantly enlarge the known cloud population. Individual T Tauri X-ray luminosities range from ∼6×10 28 to ∼2×10 31 ergs s −1 (0.4-2.5 keV), or ∼10 2 -10 4 times solar levels. The ROSAT images are a order of magnitude more sensitive, with 3-4 times more stellar identifications, than earlier Einstein Observatory images of the cloud

ROSAT X-Ray Study of the Chamaeleon I Dark Cloud. I. The Stellar Population

We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the ρ ~ 7 × 10-4 M☉ pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, ~10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.

http://iopscience.iop.org/article/10.1086/312566/pdf

Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

We present two catalogues of galaxies in the direction of the rich cluster \\a85. The first one includes 4,232 galaxies located in a region $\\pm 1^\\circ$ from the cluster centre. It has been obtained from a list of more than 25,000 galaxy candidates detected by scanning a Schmidt photographic plate taken in the \\bj band. Positions are very accurate in this catalogue but magnitudes are not. This led us to perform CCD imaging observations in the V and R bands to calibrate these photographic magnitudes. A second catalogue (805 galaxies) gives a list of galaxies with CCD magnitudes in the V and R bands for a much smaller region in the centre of the cluster. These two catalogues will be combined with a redshift catalogue of 509 galaxies (Durret et al. 1997; astro-ph/9709298) to investigate the cluster properties at optical wavelengths (Durret et al. in preparation), as a complement to our previous X-ray studies (Pislar et al. 1997, Lima-Neto et al. 1997).

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A photometric catalogue of galaxies in the cluster Abell 85

We present two catalogues of galaxies in the direction of the rich cluster Abell 496. The first one includes 3,879 galaxies located in a region of roughly 1.3 degrees from the cluster centre. It has been obtained from a list of more than 35,000 galaxy candidates detected by scanning part of a Schmidt photographic plate taken in the b_J band. Positions are very accurate in this catalogue but magnitudes are not. This led us to perform CCD imaging observations in the V and R bands to calibrate these photographic magnitudes. A second catalogue gives a list of galaxies with CCD magnitudes in the V (239 galaxies) and R (610 galaxies) bands for a much smaller region in the centre of the cluster. These two catalogues will be combined with a redshift catalogue of 466 galaxies (Durret et al. 1999) to investigate the cluster properties at optical wavelengths (Durret et al. in preparation), as a complement to previous X-ray studies by a member of our group (Pislar 1998).

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A photometric catalogue of galaxies in the cluster Abell 496

We obtained a catalogue of 4232 galaxies in the rich cD cluster of galaxies A 85 (z=0.0556) by scanning a bJ Schmidt plate with the MAMA measuring machine in a region of ±1° ; magnitudes were recalibrated with six CCD fields in V and R. We also obtained 421 redshifts, which added to the 150 in the literature[2,6] gave a catalogue of 551 galaxies.

Jean Guibert

Papers

ROSAT X-Ray Study of the Chamaeleon I Dark Cloud. I. The Stellar Population

Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

A photometric catalogue of galaxies in the cluster Abell 85

A photometric catalogue of galaxies in the cluster Abell 496

Redshift and Photometric Survey of the X-ray Cluster of Galaxies Abell 85