LOW-MASS X-RAY BINARIES INDICATE A TOP-HEAVY STELLAR INITIAL MASS FUNCTION IN ULTRACOMPACT DWARF GALAXIES

Starburst galaxies are currently forming massive stars at prodigious rates. I discuss the star-formation histories and the shape of the initial mass function, with particular emphasis on the high- and on the low-mass end. The classical Salpeter IMF is consistent with constraints from observations of the most massive stars, irrespective of environmental properties. The situation at the low-mass end is less clear: direct star counts in nearby giant H II regions show stars down to ~1 M⊙, whereas dynamical arguments in some starburst galaxies suggest a deficit of such stars.

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The Globular Cluster Population of X-ray Binaries

Symposium - International Astronomical Union ◽

10.1017/s0074180900160711 ◽

1987 ◽

Vol 125 ◽

pp. 187-197 ◽

Cited By ~ 5

Author(s):

Frank Verbunt ◽

Piet Hut

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Mass Function ◽

Initial Mass Function ◽

Encounter Rate ◽

Formation Mechanisms ◽

Main Sequence Stars ◽

X Ray ◽

Low Mass ◽

X Ray Binaries

We discuss formation mechanisms for low-mass X-ray binaries in globular clusters. We apply the most efficient mechanism, tidal capture in close two-body encounters between neutron and main-sequence stars, to the clusters of our galaxy. The observed number of X-ray sources in these can be explained if the birth velocities of neutron stars are higher than estimated from velocity measurements of radiopulsars, or if the initial mass function steepens at high masses. We perform a statistical test on the distribution of X-ray sources with respect to the number of close encounters in globular clusters, and find satisfactory agreement between the tidal capture theory and observation, apart from the presence of low-mass X-ray binaries in four clusters with a very low encounter rate: Ter 1, Ter 2, Gr 1 and NGC 6712.EXOSAT observations indicate that some dim globular cluster sources may be less luminous than hitherto assumed, and support the view that the brighter dim sources may be soft X-ray transients in quiescence.

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

The Astrophysical Journal ◽

10.3847/1538-4357/ac2a30 ◽

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.

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Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2679 ◽

2020 ◽

Vol 498 (3) ◽

pp. 4051-4059 ◽

Cited By ~ 1

Author(s):

Timothy A Davis ◽

Freeke van de Voort

Keyword(s):

Mass Loss ◽

Cosmic Time ◽

Mass Function ◽

Stellar Mass ◽

Initial Mass Function ◽

Initial Mass ◽

Early Type ◽

Root Cause ◽

Stellar Initial Mass Function ◽

Low Mass

ABSTRACT The observed stellar initial mass function (IMF) appears to vary, becoming bottom-heavy in the centres of the most massive, metal-rich early-type galaxies. It is still unclear what physical processes might cause this IMF variation. In this paper, we demonstrate that the abundance of deuterium in the birth clouds of forming stars may be important in setting the IMF. We use models of disc accretion on to low-mass protostars to show that those forming from deuterium-poor gas are expected to have zero-age main-sequence masses significantly lower than those forming from primordial (high deuterium fraction) material. This deuterium abundance effect depends on stellar mass in our simple models, such that the resulting IMF would become bottom-heavy – as seen in observations. Stellar mass loss is entirely deuterium free and is important in fuelling star formation across cosmic time. Using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation we show that stellar mass-loss-induced deuterium variations are strongest in the same regions where IMF variations are observed: at the centres of the most massive, metal-rich, passive galaxies. While our analysis cannot prove that the deuterium abundance is the root cause of the observed IMF variation, it sets the stage for future theoretical and observational attempts to study this possibility.

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Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833055 ◽

2018 ◽

Vol 620 ◽

pp. A39 ◽

Cited By ~ 32

Author(s):

T. Jeřábková ◽

A. Hasani Zonoozi ◽

P. Kroupa ◽

G. Beccari ◽

Z. Yan ◽

...

Keyword(s):

Star Formation ◽

Dwarf Galaxies ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Probability Density Distribution ◽

Star Forming ◽

The Galaxy ◽

Stellar Initial Mass Function ◽

Stellar Masses

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, [Fe/H] ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity [Fe/H] < 0 and SFR > 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR < 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities [Fe/H] > 0 the gwIMF can become bottom heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top heavy within the first few hundred Myr of the life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα relation and provide new fitting functions. Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star-forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.

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