By Linda Smith, European Space Agency/STScI
The upper mass limit for stars is not known with any certainty. The best means of observationally determining this parameter is to study the content of young, massive star clusters. The clusters need to be young (< 2 Myr) because of the short lifetime of the most massive stars, and they need to be massive enough (> 105 Msun) to sample the full extent of the initial mass function (IMF).
In 2005, Don Figer derived an upper mass limit for stars of 150 Msunusing the Arches cluster near the center of our Galaxy. However, the Arches cluster is too old at 4 Myr to sample the true initial mass function (IMF) because stars more massive than 150 Msun will have already exploded.
In a star-forming region, the most massive stars will dominate the ionization and stellar wind feedback for the first few million years. The amount of feedback will be severely underestimated from models if the upper stellar mass cut-off of the IMF is too low. Most stellar population synthesis models, which are used to infer the stellar content and feedback of unresolved star-forming regions, adopt cut-off values of 100 or 120 Msun (e.g. Starburst99; Leitherer et al. 1999).
The massive star cluster R136 in the 30 Doradus region of the Large Magellanic Cloud (LMC) is the only nearby resolved cluster which is young and massive enough to measure the IMF, and thus empirically determine the stellar upper mass cutoff. In a series of papers, Crowther et al. (2010, 2016) used far-ultraviolet (FUV) spectra obtained with spectrographs on HST to determine the masses of the massive stars using modeling techniques. They found that the R136 cluster is only 1.5 ± 0.5 Myr old and contains eight stars more massive than 100 Msun with the most massive star (called R136a1) having a current mass of 315±50 Msun. The four most massive stars account for one-quarter of the total ionizing flux from the star cluster. These very massive stars (VMS, M > 100 Msun) have very dense, optically thick winds and their emission-line spectra resemble Wolf-Rayet (W-R) stars but they are hydrogen-rich (see the recent blog article by Tony Marston on W-R stars).
Beyond R136, the best means of finding VMS is to look for their spectral signatures in the integrated FUV light of young massive star clusters in star-forming galaxies. NGC 5253 is a blue compact galaxy with a young central starburst at a distance of 3.15 Mpc. The galaxy is part of the Legacy Extragalactic UV Survey (LEGUS; see https://legus.stsci.edu), a Cycle 21 HST large program. In a paper by Calzetti et al. (2015), we combined the LEGUS imaging with HST archive images and derived the masses and ages of the bright, young star cluster population of NGC 5253 using 13 band photometry. In Fig. 1, the LEGUS image of NGC 5253 is shown. Fig 2 shows the two clusters (numbered #5 and #11) at the center of the galaxy. Cluster #5 coincides with the peak of the Hα emission in the galaxy and cluster #11 with a massive ultracompact H II region.
Figure 1: Three color composite of the central 300 x 250 pc of NGC 5253 from Calzetti et al. (2015). The 11 brightest clusters are identified and numbered.
Figure 2: Detailed view of the two nuclear clusters #5 and #11 shown in Fig. 1, which are separated by a projected distance of 5 pc.
We found that the two nuclear clusters have ages of only 1±1 Myr and masses of 7.5 x 104 and 2.5 x 105 Msun. Interestingly, the very young ages we derive contradict the age of 3-5 Myr, inferred from the presence of W-R emission-line features in the optical spectrum of cluster #5. Could these W-R features arise from very massive stars instead? To answer this, we examined archival FUV STIS and FOS spectra and optical spectra from the Very Large Telescope (VLT) of cluster #5 to search for the spectral features of VMS. This study is described in Smith et al. (2016).
The FUV spectra show that cluster #5 does indeed have the signature of very massive stars rather than much older classical W-R stars. The FUV spectrum of cluster #5 is shown in Fig. 3 and compared to the integrated FUV STIS spectrum of R136a (Crowther et al. 2016), which has been scaled to the distance of NGC 5253. The similarity between the two spectra is striking. The crucial VMS spectral features are the presence of blue-shifted O V λ1371 wind absorption, broad He II λ1640 emission, and the absence of a Si IV λ1400 P Cygni profile (expected in W-R stars). Crowther et al. (2016) find that 95% of the broad He II emission shown in the R136a spectrum in Fig. 3 originates solely from VMS. Thus the presence of this feature in emission together with the O V wind absorption indicates a very young age (< 2 Myr) and a mass function that extends well beyond 100 Msun.
Figure 3: The HST FUV spectrum of NGC 5253 cluster #5 compared to the integrated HST/STIS spectrum of R136a (Crowther et al. 2016). The R136a spectrum has been scaled to the distance of NGC 5253. The flux is in units of 1015 erg s-1 cm-2 Å-1
The presence of very massive stars in cluster #5 (and also probably cluster #11) can also explain the very high observed ionizing flux. Previous studies have assumed an age of 3-5 Myr and find that standard stellar population synthesis codes significantly under-predict the ionizing flux. For an age of 1 Myr, the predicted ionizing flux is still too low by a factor of 2 for a standard IMF with an upper mass cut-off of 100 Msun. However, only 12 VMS with M > 150 Msunare needed to make up the deficit.
The UV spectrum of cluster #5 shows many similarities with the rest frame spectra of metal-poor, high-redshift galaxies with broad He II emission and strong O III] and C III] nebular emission lines. If VMS exist in young star-forming regions at high redshift, their presence should be revealed in the UV rest-frame spectra to be obtained by the James Webb Space Telescope. For all studies near and far, it is crucial to extend stellar population synthesis models into the VMS regime to correctly model the spectra, and account for the radiative and stellar wind feedback, which will be dominated by VMS for the first 1–3 Myr in massive star-forming regions.
Calzetti, D. et al., 2015, ApJ, 811, 75
Crowther, P.A. et al., 2010, MNRAS, 408, 731
Crowther, P.A. et al., 2016, MNRAS, 458, 624
Figer, D.F., 2005, Nature, 434, 192
Leitherer, C. et al., 1999, ApJS, 123, 3
Smith, L.J. et al., 2016, ApJ, 823:38