Sep 072016
 

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.

fig1

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.

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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.

fig3

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.

References

  • 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
Nov 032014
 

By Torsten Böker, ESA astronomer at STScI

Almost 25 years of HST observations have shown that most, if not all, large galaxies harbor a super-massive black hole (SMBH) in their nucleus. How and when these exotic objects formed is one of the puzzles of current astronomy. Many black hole studies are focused on images of the high-redshift universe because those can unveil the structure of galaxies in their distant past, and thus tackle the “chicken-and-egg” question of what came first, the black hole or the galaxy.

An alternative path to addressing this question is to study low-mass nearby galaxies, which have only very small central black holes or none at all. Why are these black holes “lagging” in their evolution? Are they still growing, and if so, what regulates their growth?

The answers most likely have to consider the presence of another type of compact massive object often found in galactic nuclei, namely an extremely massive and dense cluster of stars. In fact, these so-called “nuclear star clusters” (NSCs) are the densest stellar systems known, with many millions of stars packed in a radius of only a few light years. To measure their compact structure requires the highest possible spatial resolution, and consequently, NSCs have been studied systematically only during the last decade or so.

It has become clear that NSCs are found in nearly all low-mass galaxies, and that they are an essential ingredient for any recipe to understand the evolution of galactic nuclei. They are most easily observed in the absence of a luminous bulge, which is why our work is focused on late-type spiral galaxies. Fig. 1 shows a prototypical example of a NSC, namely the one in the nearby bulge-less spiral NGC 1042. This NSC is known to contain a low-luminosity SMBH with a mass of less than a million solar masses.

Other well-known examples for a SMBH within a NSC are NGC 4395, and of course our own Milky Way. On the other hand, the Triangulum Galaxy (Messier 33) also has a NSC and is very similar to NGC 1042 in mass and size, but it does not appear to contain any SMBH in its nucleus, at least none more massive than a few tens of thousands solar masses.

Why then do SMBHs exist within some NSCs, but not in others? What regulates the relative importance of the two types of central massive objects, i.e. the ratio of their masses? Does a SMBH destroy its parent cluster once it reaches a certain mass? Or does the presence of a NSC prevent the SMBH from growing its mass any further?

NGC1042_BV_3_mos

Figure 1: Hubble Space Telescope colour composite image of NGC 1042, created from WFPC2 F450W and F606W exposures. The structural fits to the NSC show unresolved residual emission, indicating the presence of an additional point source (from Georgiev & Böker 2014).

A systematic comparison of NSCs with and without confirmed SMBHs promises to shed light on these questions. Unfortunately, at present there are only a handful of galaxies known to host both a NSC and a SMBH, thus making a statistically sound comparison difficult. In order to improve this situation, my collaborators and I are working to develop observational methods to find more systems with coexisting NSC and SMBH. The challenge here lies in the fact that in this mass range, SMBHs are very difficult to detect, because they are much less active than their more massive counterparts.

We begin by using HST imaging to constrain the luminosities, sizes, and masses of NSCs in a large number of nearby spiral galaxies. We have recently completed a systematic analysis of all WFPC2 images in the HST archive that contain NSCs. By carefully analyzing the color and shape of the NSCs, we find some cases with point-like residual emission which may be indicative of an active galactic nucleus, and thus of a SMBH. Such residuals are, in fact, evident in NGC 1042 (see the inlays in Figure 1) – they likely are caused by the “extra” emission from the SMBH.

We then follow up these SMBH candidates with adaptive optics-assisted ground-based spectroscopy that will allow us to measure the age and total mass of the NSC, and to search for spectroscopic signs for the presence of a SMBH. On the theoretical front, we try to improve our understanding of dense stellar systems by analyzing poorly understood effects caused by the presence of large amounts of gas in the early days of NSC formation which may lead to an evolving stellar mass function via gas accretion, or to runaway growth of stellar-mass black holes.

In this way, we hope to better understand the mutual interaction of SMBHs and NSCs, and to ultimately to learn how “monster” black holes in massive galaxies have formed.

Oct 152014
 

by Van Dixon, Scientist at STScI

Optical color-magnitude diagrams of Galactic globular clusters generally include one or two UV-bright stars, objects that are brighter than the horizontal branch and bluer than the red-giant branch.  When astronomers first observed these luminous, blue stars, they were a bit confused: What are young, massive stars doing in globular clusters?  The puzzle was solved with the realization that UV-bright stars are not main-sequence stars, but pre-white dwarfs.  Objects in transition, they are evolving from the asymptotic giant branch (or directly from the blue horizontal branch) to the hot tip of the white-dwarf cooling curve.  Because this phase of stellar evolution is short, lasting 103 to 105 years, these objects are rare; only a few dozen are known.

UV-bright stars in globular clusters can be used to address a number of questions. Their high temperatures (generally 10,000 to 100,000 K) make them ideal targets for far-ultraviolet spectroscopy, a powerful tool for measuring chemical abundances, particularly of species that are unobservable in cool, red-giant-branch (RGB) stars.  To first order, globular cluster stars have identical ages and initial chemical compositions, so any differences between a UV-bright star’s chemistry and that of its RGB siblings provide important constraints on theories of post-RGB evolution.  Astronomers studying the interstellar medium use UV-bright stars as background sources (with known distances!) for absorption-line studies of the ISM.

One of the most famous (and best-named) UV-bright stars is von Zeipel 1128 in the globular cluster M3 (hereafter vZ 1128; von Zeipel 1908, Strom & Strom 1970). Pierre Chayer (STScI) and I have analyzed archival FUSE, HST/STIS, and Keck HIRES spectra of this remarkable object.  By fitting the H I, He I, and He II lines in its optical spectrum with non-LTE models, we derive an effective temperature Teff = 36,000 K, a gravity log g = 3.95, and a helium abundance N(He)/N(H) = 0.15.  By comparing absorption features in the star’s FUSE and STIS spectra with a set of synthetic spectra, we can determine its photospheric abundances of C, N, O, Al, Si, P, S, Fe, and Ni.  No features from elements beyond the iron peak are observed.

Dixon_fig1Figure 1.  Comparison of the abundances derived for vZ 1128 (filled symbols) with those of the solar photosphere (short horizontal lines) and of RGB stars in M3 (rectangles).  Stellar abundances derived without an additional source of line broadening are plotted as circles, those allowing for stellar rotation are plotted as triangles, and those allowing for microturbulence are plotted as squares.

In Figure 1, the measured abundances of vZ 1128 (solid symbols) are compared with those of the sun (short vertical lines; Asplund et al. 2009) and the RGB stars in M3 (rectangles).  Cluster C and N abundances are from Smith et al. (1996), and the O, Al, Si, Fe, and Ni values are from Sneden et al. (2004).  The vertical extent of each rectangle represents the star-to-star scatter in the measured abundance (±1σ about the mean).  Beginning with the most massive elements, we see that the abundances of Si, Fe, and Ni are nearly constant along the RGB.  The scatter is much larger for CNO and Al, reflecting well-known abundance variations in globular-cluster giants (Kraft 1994).  For all of these elements, the measured abundances of vZ 1128 are consistent with those of the RGB stars.

Dixon_fig2

Figure 2.  Abundance ratios of red giants in the globular clusters M3 (triangles) and M13 (squares).  CN-rich stars are plotted as solid symbols, CN-poor stars as open symbols.  The abundances of C and O are correlated, N and O are anticorrelated, and the total abundance of (C+N+O) is essentially constant, consistent with the products of CNO-cycle processing.  The abundance ratios of vZ 1128 in M3 (circles) are consistent with the patterns seen in the RGB stars.

In Figure 2, we reproduce a plot from Smith et al. (1996), who studied variations in the CNO abundances of RGB stars in M3 and M13, which have similar metallicities.  They found that the abundances of C and O are correlated, the N abundance is anticorrelated with both C and O, and the total abundance C+N+O is nearly constant.  These patterns can be explained as the result of CNO-cycle hydrogen burning on the RGB, which converts carbon (rapidly) and oxygen (slowly) into nitrogen, but leaves the total C+N+O abundance unchanged.  Some form of mixing brings this CNO-processed material to the surface (Gratton et al. 2000); as the star ascends the RGB, it moves from right to left in Figure 2.  Comparing abundances derived from non-LTE models of an O-type star observed in the FUV with those derived from LTE models of K-type giants observed in the optical is dangerous; nevertheless, we have added vZ 1128 to Figure 2.  Though its carbon abundance is a bit high, the star’s CNO abundances follow the trends seen in the cluster’s RGB stars remarkably well.

The effective temperature and luminosity of vZ 1128 place it on the 0.546 M post-AGB evolutionary track of Schönberner (1983).  This track traces the evolution of a star that leaves the AGB before the onset of thermal pulsing and the attendant churning of its envelope known as third dredge-up.  Such objects are called post-early AGB (post-EAGB) stars.  This scenario is consistent with the abundance pattern seen in the star’s photosphere: its carbon abundance is not enhanced, nor are any s-process elements detected.  We conclude that vZ 1128 has not undergone third dredge-up.  Indeed, it appears that no significant changes in its photospheric composition have occurred since the star left the RGB.

That vZ 1128 is not a bona fide post-AGB star is not really a surprise. Post-AGB stars evolve quickly, remaining luminous for only 103–104 years, while post-EAGB stars remain luminous for 104–105 years (Schönberner 1981, 1983). Of the roughly one dozen UV-bright stars in globular clusters whose spectra have been analyzed to date (see Moehler 2010 for a review), only two show the enhanced carbon abundance expected of a star that underwent third dredge-up. The first, K648 in M15 (Rauch et al. 2002), hosts a planetary nebula, so most certainly evolved to the tip of the AGB. The second, ZNG 1 in M5, lacks a nebula, and its high helium abundance and high rotational velocity suggest an unusual evolutionary history (Dixon et al. 2004). The dearth of carbon-rich post-AGB stars in galactic globular clusters is consistent with the short lifetimes of these rare objects.

 

 

References:

  1. Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481
  2. Dixon, W. V., Brown, T. M., Landsman, W. B. 2004, ApJ, 600, L43
  3. Gratton, R. G., Sneden, C., Carretta, E., Bragaglia, A. 2000, A&A, 354, 169
  4. Kraft, R. P. 1994, PASP, 106, 553
  5. Moehler, S. 2010, MmSAI, 81, 838
  6. Schönberner, D. 1981, A&A, 103, 119
  7. Schönberner, D. 1983, ApJ, 272, 708
  8. Sneden, C., Kraft, R. P., Guhathakurta, P., Peterson, R. C., & Fulbright, J. P. 2004, AJ, 127, 2162
  9. Smith, G. H., Shetrone, M. D., Bell, R. A., Churchill, C. W., & Briley, M. M. 1996, AJ, 112, 1511
  10. Strom, S. E., & Strom, K. M. 1970, ApJ, 159, 195
  11. von Zeipel, H. 1908, Annales de l’Observatoire de Paris, 25, F1