Article

Extreme adaptive optics astrometry of R136: Searching for high proper motion stars

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Abstract

We compared high-contrast near-infrared images of the core of R136 taken by VLT/SPHERE, in two epochs separated by 3.06 years. For the first time we monitored the dynamics of the detected sources in the core of R136 from a ground-based telescope with adaptive optics. The aim of these observations was to search for High prOper Motion cAndidates (HOMAs) in the central region of R136 ( r < 6″) where it has been challenging for other instruments. Two bright sources ( K < 15 mag and V < 16 mag) are located near R136a1 and R136c (massive WR stars) and have been identified as potential HOMAs. These sources have significantly shifted in the images with respect to the mean shift of all reliable detected sources and their neighbours, and six times their own astrometric errors. We calculate their proper motions to be 1.36 ± 0.22 mas yr ⁻² (321 ± 52 km s ⁻¹ ) and 1.15 ± 0.11 mas yr ⁻² (273 ± 26 km s ⁻¹ ). We discuss different possible scenarios to explain the magnitude of such extreme proper motions, and argue for the necessity to conduct future observations to conclude on the nature of HOMAs in the core of R136.

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... A value of = 0.5 means the embedded star cluster is in virial equilibrium, < 0.5 means contraction and > 0.5 means expansion. After an exploration of different states for the second generation of stars, we report results from = 0.3 which can reproduce closely the observations of R136 presented by Khorrami et al. (2021) (hereafter K2021). ...
... We measure the 2D mass density profiles for a given radius as it was done by K2021 and we summarise the results in Figure 7 ALL Khorrami et al. (2021) K2021 at different estimated ages. Our results are matching the curve close to the centre ( < 0.2 pc) and staying slightly above until ∼ 1 pc where again they match the solid lines. ...
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... A value of = 0.5 means the embedded star cluster is in virial equilibrium, < 0.5 means contraction and > 0.5 means expansion. After an exploration of different states for the second generation of stars, we report results from = 0.3 which can reproduce closely the observations of R136 presented by Khorrami et al. (2021) (hereafter K2021). ...
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We analyzed archival Chandra X-ray observations of the central portion of the 30 Doradus region in the Large Magellanic Cloud. The image contains 20 X-ray point sources with luminosities between 5×1032 and 2×1035 ergs s-1 (0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectral type O stars as optical counterparts. Nine of these are within ~3.4 pc of R136, the central star cluster of NGC 2070. We derive an empirical relation between the X-ray luminosity and the parameters for the stellar wind of the optical counterpart. The relation gives good agreement for known colliding-wind binaries in the Milky Way Galaxy and for the identified X-ray sources in NGC 2070. We conclude that probably all identified X-ray sources in NGC 2070 are colliding-wind binaries and that they are not associated with compact objects. This conclusion contradicts earlier studies where it was argued, using ROSAT data, that two earlier discovered X-ray sources are accreting black hole binaries. Five of the 18 brightest stars in R136 are not visible in our X-ray observations. These stars either are single, have low-mass companions, or have very wide orbits. The resulting binary fraction among early-type stars is then unusually high (at least 70%).
Article
We have analyzed Hubble Space Telescope (HST) images of the compact, luminous star cluster R136 in the LMC that were taken with the refurbished HST and new Wide Field/Planetary Camera. These images allow us to examine the stellar population in a region of unusually intense star formation at a scale of 0.01 pc. We have detected stars to 23.5 in F555W and have quantified the stellar population to an M555,0 of 0.9 or a mass of 2.8 Msun. Comparisons of HR diagrams with isochrones that were constructed for the HST flight filter system from theoretical stellar evolutionary tracks reveal massive stars, a main sequence to at least 2.8 Msun, and stars with M555,0 ≥ 0.5 still on pre-main sequence tracks. The average stellar population is fit with a 3-4 Myr isochrone. Contrary to expectations from star formation models, however, the formation period for the massive stars and lower mass stars appear to largely overlap. We have measured the IMF for stars 2.8-15 Msun in three annuli from 0.5-4.7 pc from the center of the cluster. The slopes of the IMF in all three annuli are the same within the uncertainties, thus, showing no evidence for mass segregation beyond 0.5 pc. Furthermore, the combined IMF slope, -122±006 is close to a normal Salpeter IMF. The lower mass limit must be lower than the limits of our measurements: ≤ 2.8 Msun beyond 0.5 pc and ≤ 7 Msun within 0.1 pc. This is contrary to some predictions that the lower mass limit could be as high as 10 Msun in regions of intense massive star formation. Integrated properties of R136 are consistent with its being comparable to a rather small globular cluster when such clusters were the same age as R136. From the surface brightness profile, an upper limit for the core radius of 0.02 pc is set. Within a radius of 0.4 pc we estimate that there have been roughly 20 crossing times and relaxation should be well along. Within 0.5 pc crowding prevents us from detecting the intermediate mass population, but there is a hint of an excess of stars brighter than M555,0 = -5 and of a deficit in the highest mass stars between 0.6 pc and 1.2 pc. This would be consistent with dynamical segregation.
Article
Wolf-Rayet (WR) stars are evolved massive stars with strong fast stellar winds. WR stars in our Galaxy have shown three possible sources of X-ray emission associated with their winds: shocks in the winds, colliding stellar winds, and wind-blown bubbles; however, quantitative analyses of observations are often hampered by uncertainties in distances and heavy foreground absorption. These problems are mitigated in the Magellanic Clouds (MCs), which are at known distances and have small foreground and internal extinction. We have therefore started a survey of X-ray emission associated with WR stars in the MCs using archival Chandra, ROSAT, and XMM-Newton observations. In the first paper of this series, we report the results for 70 WR stars in the MCs using 192 archival Chandra ACIS observations. X-ray emission is detected from 29 WR stars. We have investigated their X-ray spectral properties, luminosities, and temporal variability. These X-ray sources all have luminosities greater than a few times 1032 ergs s−1, with spectra indicative of highly absorbed emission from a thin plasma at high temperatures typical of colliding winds in WR+OB binary systems. Significant X-ray variability with periods ranging from a few hours up to ~20 days is seen associated with several WR stars. In most of these cases, the X-ray variability can be linked to the orbital motion of the WR star in a binary system, further supporting the colliding wind scenario for the origin of the X-ray emission from these stars.
Article
Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high spatial resolution near-IR photometry. We derive high T* for the WN stars in NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~ 2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars make a disproportionate contribution to the global budget of their host clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3 systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar masses are supported by dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted L_X of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars in such a dense environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa IMF, both clusters are consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster is either too old, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - M_init for the most luminous stars in the Arches cluster approach 200 Msun according to contemporary stellar & photometric results. The potential for stars greatly exceeding 150 Msun within metal-poor galaxies suggests that such pair-instability SNe could occur within the local universe, as has been claimed for SN 2007bi (abridged).
Article
We analyze data from the Hubble Space Telescope's Advanced Camera for Surveys of the globular cluster Omega Cen. We construct a photometric and proper-motion catalog using the GO-9442, GO-10252, and GO-10775 data sets. The 2.5- to 4-year baseline between observations yields a catalog of some 10^5 proper motions, with 53,382 high-quality measurements in a central field. We determine the cluster center to ~1-arcsecond accuracy using two different star-count methods, and a completely independent method using 2MASS images. We also determine the kinematical center of the proper motions, which agrees with the star-count center to within its uncertainty. The proper-motion dispersion of the cluster increases gradually inwards, but there is no variation in kinematics with position within the central ~15 arcsec: there is no dispersion cusp and no stars with unusually high velocities. We measure for the first time in any globular cluster the variation in proper-motion dispersion with mass along the main sequence, and find the cluster not yet to be in equipartition. Our proper-motion results do not confirm the arguments put forward by Noyola, Gebhardt & Bergmann to suspect an intermediate-mass black hole (IMBH) in Omega Cen. In Paper II we present new dynamical models for the high-quality data presented here, with the aim of putting quantitative contraints on the mass of any possible IMBH. Comment: 27 pages, 28 figures, ApJ in press. v2 includes additions in response to referee comments
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