A massive star census of the starburst cluster R136
We have carried out a comprehensive census of the most massive stars in the central parsec (4 arcsec) of the starburst cluster, R136, which powers the Tarantula Nebula in the LMC using HST/STIS. R136 is both sufficiently massive that the upper mass function is richly populated and young enough that its most massive stars have yet to explode as supernovae.
The identification of very massive stars in R136, up to 300 solar masses, raises general questions of star formation, binarity and feedback in young massive clusters.
This 39 orbit Cycle 19 (GO 12465) + Cycle 20 (GO 13052) programme (Paul Crowther, PI) with HST/STIS complements the ongoing ESO/VLT Large Programme VLT-FLAMES Tarantula Survey (Chris Evans, PI) and 30 orbit Cycle 19+21 proper motion study of 30 Doradus with WFC3 and ACS (GO 12499, Danny Lennon, PI).
The R136 star cluster dissected with HST/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters. Crowther, Caballero-Nieves, Boestrom et al. MNRAS, 458, 624. arxiv:1603.04994
The R136 star cluster dissected with HST/STIS. II. Physical properties of the most massive stars in R136. Bestenlehner, Crowther, Caballero-Nieves et al. MNRAS, 499, 1918.
The R136 star cluster dissected with HST/STIS. III. In preparation.
Hubble unveils monster stars. ESA media release (March 2016).
This census enables the mass function of very massive stars to be measured for the first time, as a result of incomplete and inadequate spectroscopy to date. It also complements our Tarantula Survey, a ground-based VLT Large Programme, by including the most massive stars that are inaccessible to ground-based visual spectroscopy due to severe crowding.
These surveys, together with existing integrated UV and optical studies will enable 30 Doradus to serve as a bona-fide template for unresolved extragalactic starburst regions.
The ultraviolet STIS spectral survey of all stars more massive than ~25 solar masses has revealed:
The bulk of the visual brightest members of R136 are O2-3 stars.
Comparison of wind velocities of early O stars with those elsewhere in the LMC and the Milky Way indicate somewhat lower velocities for R136 stars.
Physical parameter estimates reveal several dozen stars more massive than 50 solar masses, with nine stars exceeding 100 solar masses (including R136c).
We obtain a cluster mass of approximately 1.5 Myr.
We have considered the integrated UV spectrum of R136a – close to 100 per cent of the HeII 1640 flux and 32 per cent of the far-UV continuum arises from very massive stars. Prominent HeII emission in the integrated spectrum of young star clusters would favour a mass function that extends well above 100 solar masses.
Complete UV/optical scan across LMC star cluster R136 with 17 slits (width 0.2 arcsec) as shown in the figure, superimposed upon a F555W WFC3/UVIS Early Release Observation (green labels are illustrative magnitudes for the circled stars).
Due to severe crowding in cluster core, initial acquisition (+peakup) is of an isolated bright star Melnick 34. Offset positions and position angle 64 degrees (E of N) are selected to ensure that both R136a1 and a2 are included in NW1 slit.
Analysis of the optical datasets are underway, and will investigate the binary properties of the massive stars, their rotational velocities and more robust physical and wind properties.
Details of observations:
Cycle 19 (GO 12465)
Visit 01 (4 orbits): STIS/CCD (G430M/3936) SE9 to NW1 (6 April 2012 at 03.33)
Visit 07 (3 orbits): STIS/CCD (G430M/3936) NW2 to NW8 (20 October 2012 at 12.08)
Visit 02 (4 orbits): STIS/CCD (G430M/4451) SE9 to NW1 (4 April 2012 at 15.22)
Visit 08 (3 orbits): STIS/CCD (G430M/4451) NW2 to NW8 (20 October 2012 at 20.29)
Visit 03 (4 orbits): STIS/CCD (G430M/4706) SE9 to NW1 (6 April 2012 at 13.48)
Visit 09 (3 orbits): STIS/CCD (G430M/4706) NW2 to NW8 (23 October 2012 at 10.00)
Visit 04 (4 orbits): STIS/MAMA (G140L) SE9 to NW3 (7 April 2012 at 06.39)
Visit 05 (2 orbits): STIS/MAMA (G140L) NW4 to NW8 (8 April 2012 at 06.34)
Visit 06 (5 orbits): STIS/CCD (G750M/6581) SE9 to NW8 (23 October 2012 at 01.54)
Cycle 20 (GO 13052)
Visit 01 (4 orbits): STIS/CCD (G430M/4194) SE9 to NW1 (21 October 2012 at 12.16)
Visit 02 (3 orbits): STIS/CCD (G430M/4194) NW2 to NW8 (3 April 2013 at 16.12)