Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe non-universality of the low-mass end of the IMF is robust against the choice of SSP model
108
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We perform a direct comparison of two state-of-the art single stellar population (SSP) models that have been used to demonstrate the non-universality of the low-mass end of the Initial Mass Function (IMF) slope. The two publi
rate research
Read More
The observed similarities between the mass function of prestellar cores (CMF) and the stellar initial mass function (IMF) have led to the suggestion that the IMF is already largely determined in the gas phase. However, theoretical arguments show that the CMF may differ significantly from the IMF. In this Letter, we study the relation between the CMF and the IMF, as predicted by the IMF model of Padoan and Nordlund. We show that 1) the observed mass of prestellar cores is on average a few times smaller than that of the stellar systems they generate; 2) the CMF rises monotonically with decreasing mass, with a noticeable change in slope at approximately 3-5 solar masses, depending on mean density; 3) the selection of cores with masses larger than half their Bonnor-Ebert mass yields a CMF approximately consistent with the system IMF, rescaled in mass by the same factor as our model IMF, and therefore suitable to estimate the local efficiency of star formation, and to study the dependence of the IMF peak on cloud properties; 4) only one in five pre-brown-dwarf core candidates is a true progenitor to a brown dwarf.
Recent papers have found that the inferred slope of the high-mass ($>1.5$ M$_odot$) IMF for field stars in the solar vicinity has a larger value ($sim 1.7-2.1$) than the slopes ($sim 1.2-1.7$; Salpeter= 1.35) inferred from numerous studies of young clusters. We attempt to reconcile this apparent contradiction. Stars mostly form in Giant Molecular Clouds, and the more massive stars ($gtrsim 3$ M$_odot$) may have insufficient time before their deaths to uniformly populate the solar circle of the Galaxy. We examine the effect of small sample volumes on the {it apparent} slope, $Gamma_{rm app}$, of the high-mass IMF by modeling the present day mass function (PDMF) over the mass range $1.5-6$ M$_odot$. Depending on the location of the observer along the solar circle and the size of the sample volume, the apparent slope of the IMF can show a wide variance, with typical values steeper than the underlying universal value $Gamma$. We show, for example, that the PDMFs observed in a small (radius $sim 200$ pc) volume randomly placed at the solar circle have a $sim 15-30$% likelihood of resulting in $Gamma_{rm app} gtrsim Gamma+ 0.35$ because of inhomogeneities in the surface densities of more massive stars. If we add the a priori knowledge that the Sun currently lies in an interarm region, where the star formation rate is lower than the average at the solar circle, we find an even higher likelihood ($sim 50-60%$ ) of $Gamma_{rm app} gtrsim Gamma+0.35$, corresponding to $Gamma_{rm app} gtrsim 1.7$ when the underlying $Gamma= 1.35$.
The stellar initial mass function (IMF), which is often assumed to be universal across unresolved stellar populations, has recently been suggested to be bottom-heavy for massive ellipticals. In these galaxies, the prevalence of gravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IR spectra implies an excess of low-mass ($m <= 0.5$ $M_odot$) stars over that expected from a canonical IMF observed in low-mass ellipticals. A direct extrapolation of such a bottom-heavy IMF to high stellar masses ($m >= 8$ $M_odot$) would lead to a corresponding deficit of neutron stars and black holes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IR luminosity in these galaxies. Peacock et al. (2014) searched for evidence of this trend and found that the observed number of LMXBs per unit $K$-band luminosity ($N/L_K$) was nearly constant. We extend this work using new and archival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST) observations of seven low-mass ellipticals where $N/L_K$ is expected to be the largest and compare these data with a variety of IMF models to test which are consistent with the observed $N/L_K$. We reproduce the result of Peacock et al. (2014), strengthening the constraint that the slope of the IMF at $m >= 8$ $M_odot$ must be consistent with a Kroupa-like IMF. We construct an IMF model that is a linear combination of a Milky Way-like IMF and a broken power-law IMF, with a steep slope ($alpha_1=$ $3.84$) for stars < 0.5 $M_odot$ (as suggested by near-IR indices), and that flattens out ($alpha_2=$ $2.14$) for stars > 0.5 $M_odot$, and discuss its wider ramifications and limitations.
We describe the luminosity function, based on Sersic fits to the light profiles, of CMASS galaxies at z ~ 0.55. Compared to previous estimates, our Sersic-based reductions imply more luminous, massive galaxies, consistent with the effects of Sersic- rather than Petrosian or de Vaucouleur-based photometry on the Sloan Digital Sky Survey (SDSS) main galaxy sample at z ~ 0.1. This implies a significant revision of the high mass end of the correlation between stellar and halo mass. Inferences about the evolution of the luminosity and stellar mass functions depend strongly on the assumed, and uncertain, k+e corrections. In turn, these depend on the assumed age of the population. Applying k+e corrections taken from fitting the models of Maraston et al. (2009) to the colors of both SDSS and CMASS galaxies, the evolution of the luminosity and stellar mass functions appears impressively passive, provided that the fits are required to return old ages. However, when matched in comoving number- or luminosity-density, the SDSS galaxies are less strongly clustered compared to their counterparts in CMASS. This rules out the passive evolution scenario, and, indeed, any minor merger scenarios which preserve the rank ordering in stellar mass of the population. Potential incompletenesses in the CMASS sample would further enhance this mismatch. Our analysis highlights the virtue of combining clustering measurements with number counts.
We present new results regarding the companion mass-ratio distribution (CMRD) of stars, as a follow-up of our previous work. We used a maximum-likelihood-estimation method to re-derive the field CMRD power law avoiding dependence on the arbitrary binning. We also considered two new surveys of multiples in the field for solar-type stars and M dwarfs to test the universality of the CMRD. We found no significant differences in the CMRD for M dwarfs and solar-type stars compared with previous results over the common mass ratio and separation range. The new best-fit power law of the CMRD in the field, combining two previous sets of data, is $dN/dq propto q^{beta}$, with $beta=0.25pm0.29$.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
