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Register a new userOn the White Dwarf distances to Galactic Globular Clusters
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We analyze in detail various possible sources of systematic errors on the distances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting technique). We find that the unknown thickness of the hydrogen layer of white dwarfs in clusters plays a non negligible role. For reasonable assumptions - supported by the few sparse available observational constraints - about the unknown mass and thickness of the hydrogen layer for the cluster white dwarfs, a realistic estimate of the systematic error on the distance is within +-0.10 mag. However, particular combinations of white dwarf masses and envelope thicknesses - which at present cannot be excluded a priori - could produce larger errors. Contamination of the cluster DA sequence by non-DA white dwarfs introduces a very small systematic error of about -0.03 mag in the Mv/(V-I) plane, but in the Mv/(B-V) plane the systematic error amounts to ~ +0.20 mag. Contamination by white dwarfs with helium cores should not influence appreciably the WD-fitting distances. Finally, we obtain a derivative D((m-M)v)/D(E(B-V))~ -5.5 for the WD-fitting distances, which is very similar to the dependence found when using the Main Sequence fitting technique.
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Numerical and observational evidence suggests that massive white dwarfs dominate the innermost regions of core-collapsed globular clusters by both number and total mass. Using NGC 6397 as a test case, we constrain the features of white dwarf populations in core-collapsed clusters, both at present day and throughout their lifetimes. The dynamics of these white dwarf subsystems have a number of astrophysical implications. We demonstrate that the collapse of globular cluster cores is ultimately halted by the dynamical burning of white dwarf binaries. We predict core-collapsed clusters in the local universe yield a white dwarf merger rate of $mathcal{O}(10rm{),Gpc}^{-3},rm{yr}^{-1}$, roughly $0.1-1%$ of the observed Type Ia supernova rate. We show that prior to merger, inspiraling white dwarf binaries will be observable as gravitational wave sources at milli- and decihertz frequencies. Over $90%$ of these mergers have a total mass greater than the Chandrasekhar limit. If the merger/collision remnants are not destroyed completely in an explosive transient, we argue the remnants may be observed in core-collapsed clusters as either young neutron stars/pulsars/magnetars (in the event of accretion-induced collapse) or as young massive white dwarfs offset from the standard white dwarf cooling sequence. Finally, we show collisions between white dwarfs and main sequence stars, which may be detectable as bright transients, occur at a rate of $mathcal{O}(100rm{),Gpc}^{-3},rm{yr}^{-1}$ in the local universe. We find that these collisions lead to depletion of blue straggler stars and main sequence star binaries in the centers of core-collapsed clusters.
We present new spatial models and distance estimates for globular clusters (GC) and dwarf spheroidals (dSphs) orbiting our Galaxy based on RR Lyrae (RRab) stars in the Pan-STARRS1 (PS1) 3$pi$ survey. Using the PS1 sample of RRab stars from Sesar et al. (2017) in 16 globular clusters and 5 dwarf galaxies, we fit structural models in $(l,b,D)$ space; for 13 globular clusters and 6 dwarf galaxies, we give only their mean heliocentric distance $D$. We verify the accuracy of the period-luminosity (PL) relations used in Sesar et al. (2017) to constrain the distance to those stars, and compare them to period-luminosity-metallicity (PLZ) relations using metallicities from Carretta et al. (2009). We compare our Sesar et al. (2017) distances to the parallax-based textit{Gaia} DR2 distance estimates from Bailer-Jones et al. (2018), and find our distances to be consistent and considerably more precise.
Recent observations of the white dwarf (WD) populations in the Galactic globular cluster NGC 6397 suggest that WDs receive a kick of a few km/s shortly before they are born. Using our Monte Carlo cluster evolution code, which includes accurate treatments of all relevant physical processes operating in globular clusters, we study the effects of the kicks on their host cluster and on the WD population itself. We find that in clusters whose velocity dispersion is comparable to the kick speed, WD kicks are a significant energy source for the cluster, prolonging the initial cluster core contraction phase significantly so that at late times the cluster core to half-mass radius ratio is a factor of up to ~ 10 larger than in the no-kick case. WD kicks thus represent a possible resolution of the large discrepancy between observed and theoretically predicted values of this key structural parameter. Our modeling also reproduces the observed trend for younger WDs to be more extended in their radial distribution in the cluster than older WDs.
(Abridged) Using luminosities and structural parameters of globular clusters (GCs) in the nuclear regions (nGCs) of low-mass dwarf galaxies from HST/ACS imaging we derive the present-day escape velocities (v_esc) of stellar ejecta to reach the cluster tidal radius and compare them with those of Galactic GCs with extended (hot) horizontal branches (EHBs-GCs). For EHB-GCs, we find a correlation between the present-day v_esc and their metallicity as well as (V-I)_0 colour. The similar v_esc, (V-I)_0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The v_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus the cluster v_esc can be used as parameter to describe the degree of self-enrichment. The nGCs populate the same Mv vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like wCen and NGC 2419. We argue that during accretion the rh of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. Our results support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.
We have derived accurate distances to Galactic globular clusters by combining data from the Gaia Early Data Release 3 with distances based on Hubble Space telescope HST data and literature based distances. We determine distances either directly from the Gaia EDR3 parallaxes, or kinematically by combining line-of-sight velocity dispersion profiles with Gaia EDR3 and HST based proper motion velocity dispersion profiles. We furthermore calculate cluster distances from fitting nearby subdwarfs, whose absolute luminosities we determine from their Gaia EDR3 parallaxes, to globular cluster main-sequences. We finally use HST based stellar number counts to determine distances. We find good agreement in the average distances derived from the different methods down to a level of about 2%. Combining all available data, we are able to derive distances to 162 Galactic globular clusters, with the distances to about 20 nearby globular clusters determined with an accuracy of 1% or better. We finally discuss the implications of our distances for the value of the local Hubble constant.
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