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J/ApJ/448/179                  HST photometry in R136             (Hunter+ 1995)

The intermediate stellar mass population in R136 determined from Hubble Space Telescope planetary camera 2 images Hunter D.A., Shaya E.J., Holtzman J.A., Light R.M., Earl J., Oneil Jr. <Astrophys. J. 448, 179 (1995)> =1995ApJ...448..179H
ADC_Keywords: Magellanic Clouds ; Photometry ; Clusters, open Keywords: Hertzsprung-Russell diagram - Magellanic Clouds - open clusters and associations: individual (R136) - stars: evolution - stars: luminosity function, mass function Abstract: 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 0.01pc. We have detected stars to 23.5 in F555W and have quantified the stellar population to an M_555.0 of 0.9 or a mass of 2.8M. 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 2.8M, and stars with M_555.0≥0.5 still on pre-main sequence tracks. The average stellar population is fit with a 3-4Myr 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 2.8-15M in three annuli from 0.5-4.7pc 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 0.5pc. Furthermore, the combined IMF slope, -1.22±0.06, is close to a normal Salpeter IMF. The lower mass limit must be lower than the limits of our measurements: ≤2.8M beyond 0.5pc and ≤7M within 0.1pc. This is contrary to some predictions that the lower mass limit could be as high as 10M 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 core radius of 0.02pc is set. Within a radius of 0.4pc we estimate that there have been roughly 20 crossing times and relaxation should be well along. Within 0.5pc crowding prevents us from detecting the intermediate mass population, but there is a hint of an excess of stars brighter than M_555.0=-5 and of a deficit in the highest mass stars between 0.6pc and 1.2pc. This would be consistent with dynamical segregation. Objects: ---------------------------------------------------------- RA (2000) DE Designation(s) ---------------------------------------------------------- 05 38 42.8 -69 06 03 R 136 = RMC 136 = HD 38268 ---------------------------------------------------------- File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table2.dat 126 3623 The photometry of stars in the R136 region (completed with astrometry in April 2016)
Byte-by-byte Description of file: table2.dat
Bytes Format Units Label Explanations
1- 6 I6 --- Star [1/3623] Star identification number 8- 13 F6.1 pix Xpos [0/800] X pixel coordinate (1) 15- 20 F6.1 pix Ypos [0/800] Yixel coordinate (1) 22- 27 F6.2 mag Umag [10/22]?=100.00 HST/WFPC2 F336W magnitude 29- 34 F6.2 mag e_Umag [0.01/0.6]?=100.00 F336W magnitude uncertainty 36- 41 F6.2 mag Vmag [12/25]?=100.00 HST/WFPC2 F555W magnitude 43- 48 F6.2 mag e_Vmag [0.01/1.4]?=100.00 F555W magnitude uncertainty 50- 55 F6.2 mag Imag [11/24]?=100.00 HST/WFPC2 F814W magnitude 57- 62 F6.2 mag e_Imag [0.01/0.6]?=100.00 F814W magnitude uncertainty 64- 71 F8.2 mag U-V ?=100.00 F336W-F555W color 73- 80 F8.2 mag e_U-V [0.02/0.6]?=100.00 F336W-F555W color uncertainty 82- 89 F8.2 mag V-I ?=100.00 F555W-F814W color 91- 98 F8.2 mag e_V-I [0.02/1.5]?=100.00 F555W-F814W color uncertainty 102-103 I2 h RAh [5] Right Ascension J2000 (hours) (2) 105-106 I2 min RAm [38] Right Ascension J2000 (minutes) (2) 108-113 F6.3 s RAs Right Ascension J2000 (seconds) (2) 115 A1 --- DE- Declination J2000 (sign) (2) 116-117 I2 deg DEd [69] Declination J2000 (degrees) (2) 119-120 I2 arcmin DEm [5/6] Declination J2000 (minutes) (2) 122-126 F5.2 arcsec DEs Declination J2000 (seconds) (2)
Note (1): the X,Y coordinates are those in the short exposure F555W image, and the coordinate system is indicated in Figure 2 of the printed paper. Note (2): accurate positions were supplied in April 2016 by B. Skiff (Lowell Obs.); the astrometry accuracy is estimated about ±0.1arcsec, the internal error being about 0.02arcsec (0.3pix). More details about the reduction are given below in the "Astrometry" section below.
Astrometry of R136 stars, by Brian A. Skiff, Lowell Obs (2016-04-08): A straightforward quadratic fit yielded mean residuals of 0.3 pixels in each axis at an image-scale of 0".045/pixel. Thus the internal errors are only a few hundredths of an arcsecond. The coordinates for the reference stars have come solely from my reduction of the Selman et al 1999 study (J/A+A/341/98/table2), where I used a number of UCAC2 and 2MASS stars for the reference frame. Although they are tied very closely to the ICRF system, the two catalogues inevitably have some "softness" in this extremely crowded region. Clearly the small internal errors indicate the Selman catalogue is pretty good. Comparison with not-too-crowded stars appearing in the excellent IRSF (II/288), in the Parker 1992 survey (II/187, also similarly reduced by me), and other sources with less well-defined astrometry suggests the overall external adjustment is about ± 0.1" rms. However, because of the extreme crowding I leave the results to 0.01" precision, if only so that cases like R136a1 and a2 (stars 3 and 5 of the table) can be distinguished. History: * 29-Jan-1996: from AAS CD-ROM series, Volume 5, 1995 prepared by Lee Brotzman [ADS] on 10-Oct-1995 * 08-Apr-2016: accurate positions added, from B. Skiff [Lowell Obs.]
(End) [CDS] 29-Jan-1996
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