Dynamical modelling of globular clusters: challenges for the robust determination of IMBH candidates

ABSTRACT The presence or absence of intermediate-mass black holes (IMBHs) at the centre of Milky Way globular clusters (GCs) is still an open question. This is due to either observational restrictions or limitations in the dynamical modelling method; in this work, we explore the latter. Using a sample of high-end Monte Carlo simulations of GCs, with and without a central IMBH, we study the limitations of spherically symmetric Jeans models assuming constant velocity anisotropy and mass-to-light ratio. This dynamical method is one of the most widely used modelling approaches to identify a central IMBH in observations. With these models, we are able to robustly identify and recover the mass of the central IMBH in our simulation with a high-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 4{{\ \rm per\ cent}}$). Simultaneously, we show that it is challenging to confirm the existence of a low-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 0.3{{\ \rm per\ cent}}$), as both solutions with and without an IMBH are possible within our adopted error bars. For simulations without an IMBH, we do not find any certain false detection of an IMBH. However, we obtain upper limits that still allow for the presence of a central IMBH. We conclude that while our modelling approach is reliable for the high-mass IMBH and does not seem to lead towards a false detection of a central IMBH, it lacks the sensitivity to robustly identify a low-mass IMBH and to definitely rule out the presence of an IMBH when it is not there.

Download Full-text

Black dwarf supernova in the far future

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2262 ◽

2020 ◽

Vol 497 (4) ◽

pp. 4357-4362

Author(s):

M E Caplan

Keyword(s):

White Dwarfs ◽

High Mass ◽

Observable Universe ◽

Dwarf Stars ◽

Low Mass ◽

Chandrasekhar Limit ◽

Ultimate Fate ◽

Open Question ◽

The Universe ◽

Far Future

ABSTRACT In the far future, long after star formation has ceased, the universe will be populated by sparse degenerate remnants, mostly white dwarfs, though their ultimate fate is an open question. These white dwarfs will cool and freeze solid into black dwarfs while pycnonuclear fusion will slowly process their composition to iron-56. However, due to the declining electron fraction, the Chandrasekhar limit of these stars will be decreasing and will eventually be below that of the most massive black dwarfs. As such, isolated dwarf stars with masses greater than ∼1.2 M⊙ will collapse in the far future due to the slow accumulation of iron-56 in their cores. If proton decay does not occur, then this is the ultimate fate of about 1021 stars, approximately 1 percent of all stars in the observable universe. We present calculations of the internal structure of black dwarfs with iron cores as a model for progenitors. From pycnonuclear fusion rates, we estimate their lifetime and thus delay time to be 101100 yr. We speculate that high-mass black dwarf supernovae resemble accretion induced collapse of O/Ne/Mg white dwarfs while later low mass transients will be similar to stripped-envelope core-collapse supernova, and may be the last interesting astrophysical transients to occur prior to heat death.

Download Full-text

D 2 O and HDS in the coma of 67P/Churyumov–Gerasimenko

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2016.0253 ◽

2017 ◽

Vol 375 (2097) ◽

pp. 20160253 ◽

Cited By ~ 34

Author(s):

K. Altwegg ◽

H. Balsiger ◽

J. J. Berthelier ◽

A. Bieler ◽

U. Calmonte ◽

...

Keyword(s):

High Ratio ◽

High Mass ◽

Mass Star ◽

Deuterated Water ◽

Star Forming ◽

Star Forming Regions ◽

Upper Limits ◽

Rosetta Mission ◽

Low Mass ◽

Grain Surface

The European Rosetta mission has been following comet 67P/Churyumov–Gerasimenko for 2 years, studying the nucleus and coma in great detail. For most of these 2 years the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has analysed the volatile part of the coma. With its high mass resolution and sensitivity it was able to not only detect deuterated water HDO, but also doubly deuterated water, D 2 O and deuterated hydrogen sulfide HDS. The ratios for [HDO]/[H 2 O], [D 2 O]/[HDO] and [HDS]/[H 2 S] derived from our measurements are (1.05 ± 0.14) × 10 −3 , (1.80 ± 0.9) × 10 −2 and (1.2 ± 0.3) × 10 −3 , respectively. These results yield a very high ratio of 17 for [D 2 O]/[HDO] relative to [HDO]/[H 2 O]. Statistically one would expect just 1/4. Such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. The high [HDS]/[H 2 S] ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary H 2 S with presolar grain surface chemistry. This article is part of the themed issue ‘Cometary science after Rosetta’.

Download Full-text

New insights into star cluster evolution towards energy equipartition

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slab026 ◽

2021 ◽

Vol 504 (1) ◽

pp. L12-L16

Author(s):

Václav Pavlík ◽

Enrico Vesperini

Keyword(s):

Radial Velocity ◽

Globular Clusters ◽

Velocity Dispersion ◽

Tangential Velocity ◽

Star Cluster ◽

High Mass ◽

Stellar Kinematics ◽

Low Mass Stars ◽

Velocity Anisotropy ◽

Energy Equipartition

ABSTRACT We present the results of a study aimed at exploring the evolution towards energy equipartition in star cluster models with different initial degrees of anisotropy in the velocity distribution. Our study reveals a number of novel aspects of the cluster dynamics and shows that the rate of evolution towards energy equipartition (i) depends on the initial degree of radial velocity anisotropy – it is more rapid for more radially anisotropic systems; and (ii) differs for the radial and the tangential components of the velocity dispersion. (iii) The outermost regions of the initially isotropic system evolve towards a state of ‘inverted’ energy equipartition in which high-mass stars have a larger velocity dispersion than low-mass stars – this inversion originates from the mass dependence of the tangential velocity dispersion whereas the radial velocity dispersion shows no anomaly. Our results add new fundamental elements to the theoretical framework needed to interpret the wealth of recent and upcoming observational studies of stellar kinematics in globular clusters, and shed further light on the link between the clusters’ internal kinematics, their formation, and evolutionary history.

Download Full-text

The X-ray binary populations of M81 and M82

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318008190 ◽

2018 ◽

Vol 14 (S346) ◽

pp. 344-349

Author(s):

Paul H. Sell ◽

Andreas Zezas ◽

Stephen J. Williams ◽

Jeff J. Andrews ◽

Kosmas Gazeas ◽

...

Keyword(s):

Mass Transfer ◽

Globular Clusters ◽

Luminosity Function ◽

Main Sequence ◽

The Other ◽

High Mass ◽

Early Type ◽

X Ray ◽

Star Forming ◽

Low Mass

AbstractWe use deep Chandra and HST data to uniquely classify the X-ray binary (XRB) populations in M81 on the basis of their donor stars and local stellar populations (into early-type main sequence, yellow giant, supergiant, low-mass, and globular cluster). First, we find that more massive, redder, and denser globular clusters are more likely to be associated with XRBs. Second, we find that the high-mass XRBs (HMXBs) overall have a steeper X-ray luminosity function (XLF) than the canonical star-forming galaxy XLF, though there is some evidence of variations in the slopes of the sub-populations. On the other hand, the XLF of the prototypical starburst M82 is described by the canonical powerlaw (αcum ∼ 0.6) down to LX ∼ 1036 erg s−1. We attribute variations in XLF slopes to different mass transfer modes (Roche-lobe overflow versus wind-fed systems).

Download Full-text

Evolution of Globular Clusters Including a Degenerate Component

Symposium - International Astronomical Union ◽

10.1017/s0074180900043515 ◽

1988 ◽

Vol 126 ◽

pp. 665-666

Author(s):

Hyung Mok Lee

Keyword(s):

Globular Clusters ◽

Main Sequence ◽

High Mass ◽

Main Sequence Stars ◽

Compact Binaries ◽

The Core ◽

Degenerate Component ◽

Low Mass ◽

Mass Segregation ◽

Very High

Low mass X-ray sources observed in many globular clusters are usually interpreted as compact binaries with degenerate components (e.g., Hertz and Grindlay 1983). Degenerate stars can exist in globular clusters if the IMF contains a sufficiently large number of high mass stars. Since the main-sequence lifetime is a very steep function of stellar mass, most of degenerate stars can be regarded as primordial. If the typical mass of degenerate stars is higher than that of main-sequence stars, mass segregation makes the core crowded with degenerate stars. Tidally captured binaries between degenerates and main-sequence stars can abundantly form as the core density becomes very high.

Download Full-text

Search for New Physics with High Mass Tau Pairs in CDF Run II

International Journal of Modern Physics A ◽

10.1142/s0217751x05026352 ◽

2005 ◽

Vol 20 (15) ◽

pp. 3287-3289

Author(s):

Zongru Wan

Keyword(s):

Higgs Boson ◽

Cross Section ◽

Branching Ratio ◽

New Physics ◽

Mass Region ◽

High Mass ◽

Significant Excess ◽

Upper Limits ◽

Tau Pairs ◽

Low Mass

We present the results of a search for new particles decaying to tau pairs. Hypothetical particles, such as Z′ and MSSM Higgs boson A can potentially produce such pairs. The low-mass region, dominated by Z → ττ, is used as a control. No significant excess events over the estimated backgrounds is observed in the high-mass region, and we set upper limits on the cross section times branching ratio as a function of the Z′ and A mass.

Download Full-text

On the emergent system mass function: the contest between accretion and fragmentation

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3176 ◽

2020 ◽

Vol 500 (2) ◽

pp. 1697-1707

Author(s):

Paul C Clark ◽

Anthony P Whitworth

Keyword(s):

Star Formation ◽

Standard Deviation ◽

Mass Distribution ◽

Power Law ◽

Accretion Rate ◽

Mass Function ◽

High Mass ◽

New Model ◽

System Formation ◽

Low Mass

ABSTRACT We propose a new model for the evolution of a star cluster’s system mass function (SMF). The model involves both turbulent fragmentation and competitive accretion. Turbulent fragmentation creates low-mass seed proto-systems (i.e. single and multiple protostars). Some of these low-mass seed proto-systems then grow by competitive accretion to produce the high-mass power-law tail of the SMF. Turbulent fragmentation is relatively inefficient, in the sense that the creation of low-mass seed proto-systems only consumes a fraction, ${\sim }23{{\ \rm per\ cent}}$ (at most ${\sim }50{{\ \rm per\ cent}}$), of the mass available for star formation. The remaining mass is consumed by competitive accretion. Provided the accretion rate on to a proto-system is approximately proportional to its mass (dm/dt ∝ m), the SMF develops a power-law tail at high masses with the Salpeter slope (∼−2.3). If the rate of supply of mass accelerates, the rate of proto-system formation also accelerates, as appears to be observed in many clusters. However, even if the rate of supply of mass decreases, or ceases and then resumes, the SMF evolves homologously, retaining the same overall shape, and the high-mass power-law tail simply extends to ever higher masses until the supply of gas runs out completely. The Chabrier SMF can be reproduced very accurately if the seed proto-systems have an approximately lognormal mass distribution with median mass ${\sim } 0.11 \, {\rm M}_{\odot }$ and logarithmic standard deviation $\sigma _{\log _{10}({M/M}_\odot)}\sim 0.47$).

Download Full-text

Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab474 ◽

2021 ◽

Vol 503 (1) ◽

pp. 1490-1506

Author(s):

Maximilian Häberle ◽

Mattia Libralato ◽

Andrea Bellini ◽

Laura L Watkins ◽

Jörg-Uwe Pott ◽

...

Keyword(s):

Globular Clusters ◽

Velocity Dispersion ◽

Angular Resolution ◽

Hubble Space Telescope ◽

High Mass ◽

High Angular Resolution ◽

Stellar Kinematics ◽

Intermediate Mass ◽

The Core ◽

Large Telescopes

ABSTRACT We present an astrometric study of the proper motions (PMs) in the core of the globular cluster NGC 6441. The core of this cluster has a high density and observations with current instrumentation are very challenging. We combine ground-based, high-angular-resolution NACO@VLT images with Hubble Space Telescope ACS/HRC data and measure PMs with a temporal baseline of 15 yr for about 1400 stars in the centremost 15 arcsec of the cluster. We reach a PM precision of ∼30 µas yr−1 for bright, well-measured stars. Our results for the velocity dispersion are in good agreement with other studies and extend already existing analyses of the stellar kinematics of NGC 6441 to its centremost region never probed before. In the innermost arcsecond of the cluster, we measure a velocity dispersion of (19.1 ± 2.0) km s−1 for evolved stars. Because of its high mass, NGC 6441 is a promising candidate for harbouring an intermediate-mass black hole (IMBH). We combine our measurements with additional data from the literature and compute dynamical models of the cluster. We find an upper limit of $M_{\rm IMBH} \lt 1.32 \times 10^4\, \textrm{M}_\odot$ but we can neither confirm nor rule out its presence. We also refine the dynamical distance of the cluster to $12.74^{+0.16}_{-0.15}$ kpc. Although the hunt for an IMBH in NGC 6441 is not yet concluded, our results show how future observations with extremely large telescopes will benefit from the long temporal baseline offered by existing high-angular-resolution data.

Download Full-text