Brown and Black Dwarfs: their Structure, Evolution and Contribution to the Missing Mass

1978 ◽  
Vol 3 (3) ◽  
pp. 227-229 ◽  
Author(s):  
D. J. Stevenson
Keyword(s):  
Galactic Plane ◽  
Mass Density ◽  
Mass Function ◽  
Initial Mass Function ◽  
Structure Evolution ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Missing Mass ◽  
Dwarf Stars ◽  
Low Mass

According to Oort (1965), the mass density in the solar neighbourhood (inferred from the gravity component normal to the galactic plane) is between 50% and 150% greater than the mass density inferred from non-dwarf stars. One possible explanation for the “missing mass” is an overabundance of faint M-dwarfs (Weistrop 1972), but present indications are that this overabundance is either small (Weistrop 1976; Sanduleak 1976) or non-existent (Faber et al. 1976; Eggen 1976). Nevertheless, Salpeter’s initial mass function (Salpeter 1955) suggests that the total mass may be dominated by low mass stars, including masses M≤0.08M⊙ which never undergo significant hydrogen burning.

scholarly journals Low Mass Stars and White Dwarfs in NGC 6397

1995 ◽  
Vol 164 ◽  
pp. 408-408
Author(s):  
Guido De Marchi ◽  
Francesco Paresce ◽  
Martino Romaniello
Keyword(s):  
White Dwarf ◽  
Luminosity Function ◽  
Main Sequence ◽  
Dynamical Evolution ◽  
Mass Function ◽  
Initial Mass Function ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Low Mass ◽  
Counting Statistics

Deep WFPC2 images in wide bands centered at 606 and 802 nm were taken with the HST 5.6 arcminutes from the center of the galactic globular cluster NGC 6397. The images were used to accurately position ~ 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m814 ≃ mI ≃ 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m814 = 18.5 to m814 = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m814 ≃ 20 and m814 ≃ 22.5 are evident. The corresponding luminosity function increases slowly from M814 ≃ 6.5 to 8.5 as expected from ground based observations but then drops sharply from there down to the measurement limit. The corresponding mass function obtained by using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between ~ 0.25 and~ 0.15 M⊙, but drops towards the expected mass limit of the normal hydrogen burning main sequence at about 0.1 M⊙. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 M⊙ at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition.

Sources of Enrichment

1991 ◽  
Vol 9 (2) ◽  
pp. 234-239 ◽  
Author(s):  
M. A. Dopita
Keyword(s):  
Time Scale ◽  
Mass Range ◽  
Mass Function ◽  
Initial Mass Function ◽  
Main Element ◽  
Type I ◽  
Low Mass Stars ◽  
History Of ◽  
Low Mass ◽  
Stellar Sources

AbstractThe relative importance of the stellar sources contributing to the production matrix of the heavy elements up to iron is reviewed. Three main element groups may be distinguished: (a) oxygen and the alpha-process elements; (b) the iron-peak group; (c) helium, carbon and nitrogen. Each of these is produced in stars of a different mass range and in different ways, and it is shown that an examination of metallicity-metallicity relationships can be used to constrain models of the history of star formation, stellar evolution, and the initial mass function in galaxies.We can conclude that in the case of our local solar neighbourhood the initial Fe/O ratio was set by Type II supernovae, but that Type I½ supernovae were never important. Iron is produced by the Type I deflagration supernovae on a time-scale comparable to the infall time-scale of the gas. Carbon is produced by dredge-up in low-mass stars, but nitrogen is shown to be produced both in the stellar winds of massive stars, and in higher mass stars which give rise to the Type I planetary nebulae.

scholarly journals Do Low Luminosity Stars Matter?

2009 ◽  
Vol 5 (H15) ◽  
pp. 47-60
Author(s):  
María Teresa Ruiz
Keyword(s):  
White Dwarf ◽  
Galactic Plane ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Missing Mass ◽  
Low Mass ◽  
Astronomical Research ◽  
Red Giant ◽  
Large Telescopes ◽  
The Universe

AbstractHistorically, low luminosity stars have attracted very little attention, in part because they are difficult to see except with large telescopes, however, by neglecting to study them we are leaving out the vast majority of stars in the Universe. Low mass stars evolve very slowly, it takes them trillions of years to burn their hydrogen, after which, they just turn into a He white dwarf, without ever going through the red giant phase. This lack of observable evolution partly explains the lack of interest in them. The search for the “missing mass” in the galactic plane turned things around and during the 60s and 70s the search for large M/L objects placed M-dwarfs and cool WDs among objects of astrophysical interest. New fields of astronomical research, like BDs and exoplanets appeared as spin-offs from efforts to find the “missing mass”. The search for halo white dwarfs, believed to be responsible for the observed microlensing events, is pursued by several groups. The progress in these last few years has been tremendous, here I present highlights some of the great successes in the field and point to some of the still unsolved issues.

scholarly journals The Initial Mass Function of Low‐Mass Stars and Brown Dwarfs in Young Clusters

2000 ◽  
Vol 540 (2) ◽  
pp. 1016-1040 ◽  
Author(s):  
K. L. Luhman ◽  
G. H. Rieke ◽  
Erick T. Young ◽  
Angela S. Cotera ◽  
H. Chen ◽  
...  
Keyword(s):  
Brown Dwarfs ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Low Mass Stars ◽  
Low Mass ◽  
Young Clusters

scholarly journals Variation in the Stellar Initial Mass Function from the Chromospheric Activity of M Dwarfs in Early-type Galaxies

2021 ◽  
Vol 923 (1) ◽  
pp. 43
Author(s):  
Pieter van Dokkum ◽  
Charlie Conroy
Keyword(s):  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Early Type ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Chromospheric Activity ◽  
Stellar Initial Mass Function ◽  
Low Mass ◽  
The Imf

Abstract Mass measurements and absorption-line studies indicate that the stellar initial mass function (IMF) is bottom-heavy in the central regions of many early-type galaxies, with an excess of low-mass stars compared to the IMF of the Milky Way. Here we test this hypothesis using a method that is independent of previous techniques. Low-mass stars have strong chromospheric activity characterized by nonthermal emission at short wavelengths. Approximately half of the UV flux of M dwarfs is contained in the λ1215.7 Lyα line, and we show that the total Lyα emission of an early-type galaxy is a sensitive probe of the IMF with a factor of ∼2 flux variation in response to plausible variations in the number of low-mass stars. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure the Lyα line in the centers of the massive early-type galaxies NGC 1407 and NGC 2695. We detect Lyα emission in both galaxies and demonstrate that it originates in stars. We find that the Lyα to i-band flux ratio is a factor of 2.0 ± 0.4 higher in NGC 1407 than in NGC 2695, in agreement with the difference in their IMFs as previously determined from gravity-sensitive optical absorption lines. Although a larger sample of galaxies is required for definitive answers, these initial results support the hypothesis that the IMF is not universal but varies with environment.

scholarly journals Transition of the initial mass function in the metal-poor environments

2021 ◽  
Author(s):  
Sunmyon Chon ◽  
Kazuyuki Omukai ◽  
Raffaella Schneider
Keyword(s):  
Star Formation ◽  
Mass Distribution ◽  
Massive Stars ◽  
Mass Function ◽  
Initial Mass Function ◽  
Initial Mass ◽  
Low Mass Stars ◽  
Filamentary Structure ◽  
Low Mass ◽  
Cloud Core

Abstract We study star cluster formation in a low-metallicity environment using three dimensional hydrodynamic simulations. Starting from a turbulent cloud core, we follow the formation and growth of protostellar systems with different metallicities ranging from 10−6 to 0.1 Z⊙. The cooling induced by dust grains promotes fragmentation at small scales and the formation of low-mass stars with M* ∼ 0.01–0.1 M⊙ While the number of low-mass stars increases with metallicity, when Z/Z⊙ ≳ 10−5. the stellar mass distribution is still top-heavy for Z/Z⊙ ≲ 10−2 compared to the Chabrier initial mass function (IMF). In these cases, star formation begins after the turbulent motion decays and a single massive cloud core monolithically collapses to form a central massive stellar system. The circumstellar disk preferentially feeds the mass to the central massive stars, making the mass distribution top-heavy. When Z/Z⊙ = 0.1, collisions of the turbulent flows promote the onset of the star formation and a highly filamentary structure develops owing to efficient fine-structure line cooling. In this case, the mass supply to the massive stars is limited by the local gas reservoir and the mass is shared among the stars, leading to a Chabrier-like IMF. We conclude that cooling at the scales of the turbulent motion promotes the development of the filamentary structure and works as an important factor leading to the present-day IMF.

Circumstellar disk fragmentation and the origin of massive planetary companions, brown dwarfs, and very low-mass stars

2018 ◽  
Vol 14 (S345) ◽  
pp. 239-240 ◽  
Author(s):  
M. B. N. Kouwenhoven ◽  
Yun Li ◽  
D. Stamatellos ◽  
S. P. Goodwin
Keyword(s):  
Binary Systems ◽  
Brown Dwarfs ◽  
Mass Range ◽  
Mass Function ◽  
Initial Mass Function ◽  
Low Mass Stars ◽  
Triple Systems ◽  
Low Mass ◽  
Viable Mechanism

AbstractThe low-mass end of the initial mass function remains poorly understood. In this mass range, very low-mass stars, brown dwarfs, and massive planets are able to form through a variety of physical processes. Here, we study the long-term evolution of disk-fragmented systems around low-mass stars, for the epoch up to 10 Myr (the typical lifetime of an embedded cluster) and up to 10 Gyr (the age of the Milky Way). We carry out N-body simulations to study the decay of disk-fragmented systems and the resulting end products. Our simulations indicate rapid decay and frequent physical collisions during the first 10 Myr. We find that disk fragmentation provides a viable mechanism for explaining hierarchical triple systems, the brown dwarf desert, single and binary brown dwarfs, and very low-mass binary systems in the solar neighbourhood.

scholarly journals IMF radial gradients in most massive early-type galaxies

2019 ◽  
Vol 489 (3) ◽  
pp. 4090-4110 ◽  
Author(s):  
F La Barbera ◽  
A Vazdekis ◽  
I Ferreras ◽  
A Pasquali ◽  
C Allende Prieto ◽  
...  
Keyword(s):  
Stellar Population ◽  
Velocity Dispersion ◽  
Mass Density ◽  
Initial Mass Function ◽  
Early Type ◽  
Low Mass Stars ◽  
Surface Mass ◽  
Low Mass ◽  
Surface Mass Density ◽  
The Imf

ABSTRACT Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and near-infrared initial mass function (IMF)-sensitive features in a representative sample of galaxies at the very high mass end of the galaxy population. The sample consists of seven early-type galaxies (ETGs) at z ∼ 0.05, with central velocity dispersion in the range 300 ≲ σ ≲ 350 km s−1. Using state-of-the-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiO and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to ∼4 kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $\rm \sim 10^{10}\, M_\odot \, kpc^{-2}$, or, alternatively, within a characteristic radius of ∼2 kpc. Radial distance, in physical units, and surface mass density are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter that fully explains variations in the stellar IMF, but IMF radial profiles at z ∼ 0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.

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