VLT/FORS Surveys of Wolf-Rayet Stars Beyond the Local Group: Type Ib/C Supernova Progenitors?

Astronomical Science

VLT/FORS Surveys of Wolf-Rayet Stars beyond the Local Group: Type Ib/c Supernova Progenitors?

Paul A. Crowther, Lucy J. Hadfield unless hydrogen-rich material is mixed external galaxies. Notably, narrowband (University of Sheffield, United Kingdom) downwards from the outer zones. interference filter techniques have been independently developed by Moffat and Once the core hydrogen is exhausted, the Massey that permit their detection from Wolf-Rayet (WR) stars are the chemical- star leaves the main sequence and be- their strong emission lines at He ii λ 4686 ly evolved descendents of O stars, comes a blue supergiant, and ultimately a (WN stars) and C iii λ 4650 (WC stars) such that they trace massive star forma red supergiant (RSG) for stars with initial with respect to their nearby continuum. tion. Here we present results of recent mass up to perhaps 20–30 MA. Observa- Such techniques have been applied to VLT/FORS surveys of WR stars in near- tionally, there is an absence of luminous regions of the Milky Way disc, the Magel- by spiral and irregular galaxies and RSGs, known as the Humphreys-David- lanic Clouds and other Local Group gal- consider individual WR stars as progen- son limit, such that initially more massive axies. itors of Type Ib/c core-collapse super- stars circumvent the RSG phase, pass novae. Young massive clusters hosting through a Luminous Blue Variable stage, It is well established that the absolute large WR populations may be used as before ending their life as Wolf-Rayet number of WR stars and their subtype templates for high-redshift Lyman break (WR) stars, exhibiting either the products distribution are metallicity dependent. galaxies. of core-H burning (WN subtypes) or sub N(WR)/N(O) ~ 0.15 in the relatively metal- sequent core-He burning (WC, WO sub- rich Solar Neighbourhood, yet N(WR)/ types). N(O) ~ 0.01 in the metal-deficient SMC on Massive stellar evolution the basis of only 12 WR stars versus Consequently, the prime candidates for ~ 1000 O stars (Crowther 2007). This ob- Massive stars form in star clusters with- core-collapse SN are RSG and WR stars servational dependence follows since in star-forming galaxies, pollute the in for H-rich (Type II) and H-poor (Type Ib/c) the hydrogen-rich envelope of O stars are terstellar medium, inject energy and mo- cases, respectively. Indeed, within the more easily removed at high metallicity. mentum via powerful stellar winds and past few years a direct connection has O-type stars possess strong winds that core-collapse supernovae (SNe). The de- been established between certain Type Ic are driven by metallic lines (primarily CNO tection of massive stars within Lyman- SNe and gamma-ray bursts (GRBs), and Fe-peak elements), for which the break galaxies at high redshift, either di- supporting the collapsar model in which empirical dependence upon metallicity Z · rectly via their UV continua and indirectly the GRB results from the death throes of is M ∝ Z 0.8 for stars between SMC via ionised H ii regions provides some of a rapidly rotating WR star. and Milky Way metallicities (Mokiem et al. the most stringent constraints upon their 2007). physical properties. Relative to lower-mass stars, the evolution of high-mass stars is complicated by Attempts have been made with 4-m The Initial Mass Function favours the for- (a) the metallicity dependence of their ra- telescopes to extend the interference fil- mation of low- and intermediate-mass diatively line-driven stellar winds, produc- ter technique to star-forming galaxies stars with respect to high-mass stars, for ing weaker winds at low metallicity, and beyond the Local Group, although this which the boundary is conventionally (b) their initial rotational velocities, provid- proved to be challenging (e.g. Testor and set at 8 MA – the division between stars ing rotationally induced mixing within their Schild 1993). The advent of efficient multi- ultimately forming a CO white dwarf or interiors. It is only within the past decade object spectrographs such as FORS1/2 an iron core, the latter subsequently un- that allowance for both effects have been at the Very Large Telescope has permit- dergoing a core-collapse SNe. Spectro- considered within evolutionary models, ted surveys of WR populations in galaxies scopically, stars with initial masses of most recently implemented into spectral at distances beyond 2 Mpc (Table 1). An

8–20 MA are B-type dwarfs on the main synthesis models (Vazquez et al. 2007). example of our FORS1 imaging approach sequence, or O-type dwarfs at higher is presented in Figure 1 for the barred initial mass. Such high-mass stars pos- spiral galaxy NGC 1313 (see also the cov- sess convective cores, and radiative Surveys of Wolf-Rayet stars in star-form- erpage of the current Messenger). The envelopes, a situation reversed in the Sun ing spiral galaxies boxed region 60 × 60 arcsec in size with- and other low-mass stars. Although there in NGC 1313 hosts in excess of twen- is energy transport from the convective WR stars exhibit a unique, broad emis- ty WR stars, most of which are within a and radiative regions, only the convective sion line spectral appearance which pro- large giant H ii region. Overall, the suc- core participates in nuclear reactions, vides the basis for their detection in cess rate of identifying WR stars from

Galaxy D (Mpc) log (O/H)+12 N (O7V) N(WN) N(WC) Reference Table 1: Summary of southern star- NGC 300 1.9 8.6: 800 ≥ 16 ~ 15: Schild et al. 2003; Crowther et al. 2007 forming galaxies whose WR populations have been surveyed with NGC 1313 4.1 8.2 6 500 ≥ 51: ~ 33: Hadfield and Crowther 2007 FORS1/2 to date. The O star contents M83 4.5 9.0? 40000 ≥ 470 ≥ 560 Hadfield et al. 2005 are based upon a SFR for an assumed NGC 3125 11.5 8.4 3600 200 40 Hadfield and Crowther 2006 O7 v Lyman continuum flux of 1049 per second, omitting regions excluded from our WR surveys, i.e. the outer disc of NGC 300 and the nuclear starburst of M83.

The Messenger 129 – September 2007 53 Astronomical Science Crowther P. A. and Hadfield L. J., VLT/FORS Surveys of WR Stars beyond the LG

2005). This observational trend led to suggestions that early-type WC stars are richer in carbon than late-type WC stars. However, quantitative analysis of WC subtypes allowing for radiative transfer effects do not support a subtype dependence of elemental abundances in WC stars.

In contrast, Crowther et al. (2002) pro- posed that late spectral types follow in metal-rich environments, and early types at low metallicity due to metallicity dependent WR winds. Indeed, WO stars (extreme WC early-types) are preferentially seen at low metallicity. Consequent- ly, the representative WC subtype of a galaxy permits an estimate of its metallicity. Metal-poor WR stars possess harder Lyman continuum ionising flux distribu- tions than high metallicity counterparts, in agreement with the association of neb ular He ii λ 4686 with low-metallicity WR stars (e.g. Hadfield and Crowther 2007).

Evolutionary models for the Wolf-Rayet stage have typically assumed metallicity independent mass-loss rates, which both observational and theoretical evidence now challenges. The metallicity dependence of WN winds appears to be similar to O stars, with a somewhat weaker dependence for WC stars due to their high carbon and oxygen abundances, of rele- vance to the observed ratio of WC to WN stars predicted by evolutionary models (Eldridge and Vink 2006).

One related topic involves the search B Net HeII Net Hα for Wolf-Rayet stars in close binary systems with neutron-star or black-hole Figure 1: The upper panel shows a composite the Solar Neighbourhood. In contrast, companions. Such systems represent a FORS1 image of NGC 1313 (ESO Press Photo WN stars exceed WC stars by a factor of natural, though rare, end state for close 43a/06) showing a 60 × 60 arcsec box, for which the lower panels show FORS1 images of broadband B, ~ 5 and ~ 10 for the LMC and SMC, re- binary evolution, for which Cyg X-3 in the continuum subtracted He ii 4686 (excesses are spectively (Crowther 2007). At low metal- Milky Way has been the sole example shown in white), continuum subtracted Ha, from left- licity the reduced WR population and up until recently. The combination of high to-right. the relative dominance of WN subtypes spatial resolution X-ray surveys plus our most likely results from the metallicity de- WR surveys of nearby galaxies has in- our FORS surveys is high. For the case pendence of winds from their evolution- creased this number to three, IC 10 X-1 in of NGC 1313 we have identified 94 can ary precursors. Consequently, only the the northern sky, and NGC 300 X-1 in the didate WR stars, for which a subset most massive single stars reach the WR south, both of which are probable WR of 83 % have been spectroscopically ob- phase in metal-poor environments. plus black-hole systems (e.g. Crowther et served. Within this subset, 90 % of the al. 2007). sources have been spectroscopically Not all WR subtypes are observed in all confirmed as WR stars, as indicated in environments. Early-type WN and WC Figure 2 (Hadfield and Crowther 2007). subtypes dominate in metal-poor galaxies, such as the SMC, while late WC stars Regarding WN and WC subtype distribu- are more common at super-Solar met tions, similar numbers are observed in allicities, such as M83 (Hadfield et al.

54 The Messenger 129 – September 2007 Type Ib/c supernova and gamma-ray 7 Figure 2: Representative early-type burst progenitors? WN (upper panel) and WC (lower ] ] I I II 6 II panel) stars identified in our NGC 1313 [O [O survey (Hadfield and Crowther 2007), At solar metallicity, stars initially more together with average LMC template

) 5 II –1 β

Å spectra (shown in red) from Crowther massive than ~ 25 M apparently end He A H –2 and Hadfield (2006). their lives as either a nitrogen-rich (WN) cm 4 –1 s II V NGC 1313 #21 or carbon-rich (WC) WR star. WN and g N er He E(B–V) = 0.29

WC stars are believed to be the immedi- –1 3 0 (1 ate progenitors for a subset of Type Ib

Flux NGC 1313 #33 (H-poor) and Type Ic (H and He-poor) su- 2 E(B–V) = 0.29 pernovae, respectively. Alternatively, lower-mass binaries may produce Type Ib 1

(Type Ic) SN in which H (both H and He) WN2–4 LMC template 0 has been stripped away due to Roche 4600 4800 5000 5200 5400 5600 5800 6 000 lobe overflow and/or common envelope Wavelength (Å) evolution. Some Type Ib/c SN do occur in elliptical/S0 host galaxies – explicitely 4 2.5 from 50 Type Ib/c within the last decade within 50 Mpc – in favour of such lower- C III/HeII 2.0 mass progenitors. Nevertheless, the vast ) –1 Å majority are preferentially in actively star- –2 1.5 forming galaxies, so are most Type Ib/c cm NGC 1313 #10 –1

s E(B–V) = 0.29 C IV

SN from massive WR stars or lower-mass g er

interacting binaries? –1

0 1.0 (1

To date, broadband surveys of local Flux NGC 1313 #81 (≤ 10 Mpc) star-forming galaxies have 0.5 E(B–V) = 0.29 been undertaken with Hubble Space

Telescope and ground-based 8-m tele- WC4 LMC template 0.0 scopes by groups in the UK and US. 4600 4800 5 000 5200 5400 5600 5800 6000 These have been successful in identifying Wavelength (Å) RSG progenitors of the most common core-collapse SN (Type II-P, Smartt et al. of 4.5 Mpc. If we are able to study ~ 10 2003). These favour the ‘collapsar’ sce- 2004). Unfortunately, WR stars are vis nearby galaxies with such a high SFR we nario, involving the core collapse of a ro- ually much fainter than RSG and may not would sample in excess of 104 WR stars tating Wolf-Rayet star to a black hole via be distinguished from blue supergiants such that statistically one would be an accretion disc, in which the rotation- on the basis of existing broadband sur- expected to collapse to a Type Ib/c SN al axis provides a preferred direction for veys alone. Observationally, SN 2002ap within the next decade given that WR the jet (MacFadyen and Woosley 1999). (Type Ic) in M74 so far provides the most lifetimes are ~ 105 yr. Indeed, we have al- Hammer et al. (2006) identify WR stars stringent constraints upon a potential ready come close to success. Our FORS1 within a giant H ii region of ESO 184–G82 WR progenitor, revealing an upper limit of narrowband imaging of the merging several hundred pc from the location of

MB = –4.2 mag (Crockett et al. 2007). ‘Antennae’ galaxies (NGC 4038/9) for WR SN 1998bw = GRB 980425, and propose The precursor of SN 2002ap was either a stars was obtained in June 2005, sadly that the GRB progenitor was ejected from relatively faint WC star, or more likely a six months after the Type Ic SN 2004gt. the WR cluster. lower-mass binary system. Of course, astrometry is rather challeng- Rotation is critically important since the One application of our WR surveys with ing from ground-based imaging. High collapsar model involves highly collimated the VLT would be to establish whether spatial resolution imaging helps greatly, jets produced along the polar axes, due a WR star was the progenitor of a future as indicated in Figure 3 where we com- to a dense, equatorial accretion disc Type Ib/c SN, if it occurred in one of our pare the location of a (rare) WO star in feeding the central black hole. At low me- surveyed galaxies. For the case of M83, NGC 1313, identified from FORS1 nar- tallicity, the mechanical mass-loss in- we have identified in excess of 1000 WR rowband imaging and spectroscopic fol- duced spin-down during the Wolf-Rayet stars within the galactic disc. The star- low-up, to HST/ACS broadband imaging. phase may be avoided due to their re formation rate (SFR) in M83 may be latively weak winds, resulting in sufficient estimated from its global Ha flux of 7 × This general topic has received renewed angular momentum in the core upon core 10 –11 erg s–1 cm–2 (Kennicutt, priv. interest since a number of nearby, bright collapse. Of course, only a tiny fraction comm.), corrected for average extinction Type Ic SNe have been observationally of SNe produce a GRB, with an apparent –1 (EB–V ~ 0.5 mag), i.e. a SFR of 4.3 MA yr associated with several nearby GRBs (e.g. bias towards metal-poor environments (Kennicutt 1998) for an adopted distance SN 2003dh = GRB 030329 Hjorth et al. (Modjaz et al. submitted) with respect to

The Messenger 129 – September 2007 55 Astronomical Science Crowther P. A. and Hadfield L. J., VLT/FORS Surveys of WR Stars beyond the LG

to a spatial scale of ~ 10 pc. Relatively cases, most notably a young massive isolated WR stars have been identified, cluster within NGC 3125, alias Tol 3 albeit in the minority (recall Figure 3). This (Chandar et al. 2004). This LMC-metallic- is even more problematic for more dis- ity galaxy is dominated by a central star- tant galaxies such as M 83 where the burst region which consists of two main great majority of WR stars are observed emission knots, NGC 3125-A and -B, in clusters or associations (Hadfield et al. shown in Figure 4. From UV spectros- 2005). copy, Chandar et al. (2004) estimated 5 000 WR stars for a cluster within knot So-called ‘WR galaxies’ are typically star- A, with a remarkable N(WR)/N(O) ≥ 1. In burst regions exhibiting spectral features contrast, optical studies of NGC 3125-A from tens, hundreds, or even thousands infer a WR population that is an order of WR stars. Indeed, such knots of star of magnitude lower, in better agreement formation, hosting young massive clus- with the relative massive star content 6 ters (~ 10 MA) often reveal significant WR of other local starburst galaxies. If NGC populations, seen at UV or visual wave- lengths. Their appearance is reminiscent Figure 4: Composite B, V, Ha HST/ACS image of of the composite rest-frame UV spectrum NGC 3125 (20 × 20 arcsec = 1 × 1 kpc) in which of z ~ 3 star-forming galaxies (~ 1010 –11 M ) young massive clusters within knot A (upper right) A and knot B (lower left) host significant WR popu which also show broad He ii λ 1640 emis- lations (Hadfield and Crowther 2006). Insets are sion (Shapley et al. 2003). composite U, V and I ACS images for the central 1 × 1 arcsec for each knot, revealing clusters A1-2 A recent ultraviolet HST/STIS survey and B1-2. Our study, based upon VLT/FORS1 imaging and spectroscopy resolves previously in- of local starburst galaxies also revealed consistent massive stellar populations for cluster strong He ii λ 1640 emission in a few A1 from UV (HST/STIS) versus optical diagnostics.

A2 Figure 3: Comparison of a 10 × 10 arcsec (200 × 200 pc) region of NGC 1313 centred upon a WO star imaged by FORS1 (He ii λ 4686 filter, top) and HST/ACS (F435W/WFC, bottom). The WO has a

F435W magnitude of 23.4 mag, suggesting MF435W ~ –5.2 mag for a distance modulus of 28.0 mag

(4.1 Mpc) and AF435W ~ 0.6 mag. non-GRB Type Ic SN. As such, GRBs would trace the low-metallicity star- formation history of the Universe. The low metallicity bias favours the single sce- nario, with respect to alternative close binary models.

WR clusters as templates for Lyman break galaxies

Individual WR stars may, in general, be B2 resolved in Local Group galaxies from ground-based observations, whilst the likelihood of contamination by nearby B1 sources increases at larger distances. For example, a typical slit width of 1? at the 2 Mpc distance of NGC 300 corresponds

56 The Messenger 129 – September 2007 3125-A is an analogue for Lyman-break 7 Figure 5: De-reddened, continuum galaxies, one must be able to reconcile subtracted VLT/FORS1 spectroscopy NGC 3125-A1 of NGC 3125-A1 (black, Hadfield and ) 6 optical and UV diagnostics for this star- –1 N(WN) = 105 Crowther 2006), together with scaled Å burst galaxy. N(WC) = 20 LMC-metallicity templates for WN –2 5 (green) and WC (red) stars, plus their cm sum (blue). Hadfield and Crowther (2006) re-inves –1 4 s tigated the massive stellar content of g er 3 NGC 3125 from FORS1 imaging and 5 –1 spectroscopy, supplemented by archival 0 (1 HST imaging and spectroscopy. FORS1 2 narrowband imaging confirms that the Flux 1 NGC 3125-A and -B knots represent the primary sites of WR stars. HST imaging 0 resolves each knot into two young mas- 4550 4600 4650 4700 4750 4800 sive clusters, as shown in Figure 4. Our Wavelength (Å) FORS1 imaging reveals that both clusters within knot A host WR stars (A1 and A2), 1.4 for which the visually fainter cluster A2 is NGC 3125-A1

) 1.2 heavily reddened. From ground-based –1 N(WC) = 20 Å imaging is not clear whether B1 or B2 (or –2 1.0 both) host WR stars, since their separa- cm

tion is ~ 0.2 arcsec (10 pc). –1 0.8 s g er 0.6 LMC template WN and WC spectra 6 –1 from Crowther and Hadfield (2006) were 0 (1 matched to the FORS1 visual WR fea- 0.4

tures in A1 and B1+B2, permitting their Flux 0.2 relative contributions to be determined. From Figure 5, we derive N(WN) ~ 105 in 0.0 cluster A1, a factor of ~ 3 lower than 5700 5750 5800 5850 5900 5950 previous optical studies, owing to a lower Wavelength (Å) nebular-derived interstellar reddening. Using Starburst99 theoretical energy dis- tributions to estimate O star popula-tions for each cluster, we find N(WR)/N(O) = 0.1–0.2 for clusters A1-A2 (each 2 × 5 10 MA), in broad agreement with evolutionary models. fluence on evolutionary synthesis models References for massive stars, WR stars are Type Ib/c Chandar R. et al. 2004, ApJ 604, 153 Archival HST/STIS UV spectroscopy con- supernova and GRB progenitors, and Crockett R.M. et al. 2007, MNRAS, in press, firms the low interstellar extinction to- local young massive clusters as tem- arXiv:0706.0500 wards A1, assuming a SMC-like redden- plates for high-redshift star-forming gal- Crowther P. A. 2007, ARA&A 45, 177 ing law. We obtain N(WN) = 110 from the axies. In the near future we aim to extend Crowther P. A., and Hadfield L. J. 2006, A&A 449, 711 slit loss corrected He ii λ 1640 line flux, our surveys to other galaxies with high Crowther P. A. et al. 2002, A&A 392, 653 in excellent agreement with optical re- SFR’s in the southern hemisphere with Crowther P. A. et al. 2007, A&A 469, L31 sults. Our HST/STIS result is however a the VLT and in the northern hemisphere Eldridge J. and Vink J. 2006, A&A 452, 295 factor of 35(!) times lower than that in- with other facilities. Indeed, WFPC2 Hadfield L. J. and Crowther P. A. 2006, MNRAS 368, 1822 ferred from the same data set by Chandar (until SM4) and WF3 (after SM4) aboard Hadfield L. J. and Crowther P. A. 2007, MNRAS, et al. (2004). The discrepancy is primari- HST possess a narrow He ii filter. These in press, arXiv:0708.2039 ly due to an anomalously high far-UV-de- may be used with broadband continuum Hadfield L. J. et al. 2005, A&A 439, 265 rived extinction from their use of the ge- filters to identifty WR candiates at high Hammer F. et al. 2006, A&A 454, 103 Hjorth J. et al. 2003, Nature 423, 847 neric starburst law. Consequently, far-UV- spatial resolution. Kennicutt R. C. 1998, ARA&A 36, 189 based results for other nearby starburst MacFadyen A. and Woosley S. 1999, ApJ 524, 262 knots should be treated with caution. Mokiem M. R. et al. 2007, A&A, in press, Acknowledgements arXiv:0708.2042 Schild H. et al. 2003, A&A 397, 859 In conclusion, our ongoing WR surveys Thanks to Hans Schild for the introduction to WR Shapley A. et al. 2003, ApJ 347, 127 of nearby galaxies permit studies of surveys beyond the Local Group, ESO for including Smartt S. et al. 2004, Sci 303, 499 young massive stellar populations across Heii filters within the standard filter set of FORS, and Testor G. and Schild H. 1993, The Messenger 72, 31 a range of ambient metallicities, with in- the support of the OPC sub-panels since Period 65. Vazquez G. A. et al. 2007, ApJ 663, 995

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