Quick viewing(Text Mode)

Consequences of Very Low Metallicity for Massive Stars and the Galaxies They Inhabit /C

Consequences of Very Low Metallicity for Massive Stars and the Galaxies They Inhabit /C

Consequences of Very Low for Massive and the they Inhabit /C

Sally Heap (Goddard Emerita) and Ivan Hubeny (University of Arizona, retired)

Strong heating is evident in the -sensitive [O III] 4363Å line (Fig. 5). Thus, a Introduction Low-metallicity massive stars (M>40 M⊙) will form massive stellar At a million degrees at its inner edge, the black-hole disk adds a hard component to black holes by direct collapse the radiation of I Zw 18-NW, which is suppressed in the EUV by absorption by H, He, and stellar is the answer to question, “What Heats the Bright H II Regions in I Zw 39 18?” (Schaerer & Stasinska 99). The strong ionizing effect of the black hole is seen most Very low metallicity has important effects on the properties and evolution of massive stars and the . Fig. 3 shows the SED’s for I Zw 18 for the case of no black hole, BH Lx=1x10 erg/s, 40 vividly in He II recombination emission at 4686 Å and 1640 Å (Fig. 5, 6). Less noticeable galaxies they inhabit. Prime among the effects is the reduced strength of radiation-driven stellar and BH Lx=1x10 erg/s. According to Heger et al. (2003), at stellar death, stars less massive than 40 M⊙ will explode as but just as important is the alteration of the line flux of strong [O III] lines used for . Here, we examine the implications of a very weak or non-existent stellar of low- supernovae with perhaps a burst of gamma rays. However, stars with masses in the range ~40 – abundance analysis. Hard radiation from the accretion disks strengthens some emission metallicity massive stars (Minit > 40 M⊙): 140 M⊙ will collapse directly to a black hole without an explosion and thus, no natal kick. Hence, it lines (Figs. 5, 6) leading to an overestimate of their abundance; while emission filling in • Altered initial mass-final mass relation (Mi-Mf) of massive stars with more massive stars at the is expected that massive stars of very low metallicity will produce massive stellar black holes. some UV resonance absorption lines (Fig. 6) leads to an underestimate of the abundances end-points of their evolution; The variety of stellar remnats is shown below in Figure 2. Heger did not account for of various metals (c.f. Lebouteiller+13, James+15). • Retention of original rather than loss to a ; loss, so in general, the x-axis should be labeled “Final Mass” . However in the case of very low • End-point of is direct collapse to a black hole (BH) rather than a metallicity, stellar mass loss is insignificant, so the final mass is nearly as large as the initial mass. • Production of a massive ; • High level of photo-ionization and heating of the surrounding gas by the BH ; • Weaker or non-existent a galactic outflow.

We will apply these theoretical predictions to the blue compact dwarf , I Zw 18-NW, which has a very low metallicity (log O/H + 12 = 7.2). While galaxies with such low metallcity are rare in the low- , they are likely to be common in galaxies at cosmic dawn. Thus, the spectra of I Zw 18 is a “living” model for z>6 galaxies. The goal now is to to build theoretical models that reproduce their observed spectra. Such models can then be used as starting points to explore the physical conditions in which stars and black holes form and evolve in an extremely low-metallicity environment.

Very low-metallicity massive stars retain most of their mass

Stellar winds are driven by the transfer of momentum of stellar radiation to the surrounding gas mainly by absorption by lines. Hence, the rate of mass-loss depends on the metallicity, particularly the abundance of . As shown below, a strong rate of mass-loss can have severe consequences to the relationship between the initial mass and final mass relationship. It shows Fig. 2—Remnants of massive single stars as a function of initial mass Figure 3. CLOUDSPEC output model SED’s that massive stars with a low metallicity like that of I Zw 18 retain nearly all the mass that they and metallicity. (Heger et al. 2003). were born with. In contrast, stars with a solar metallicityIn lose most of their mass as they evolve. The main forms of feedback from the black-hole accretion disk are heating and ionization. At Modern methods of estimating the mass-loss rate that account for clumping in the wind lead to L =1x1039 erg/s, the black hole in I Zw 18-NW substantially elevates the temperature in the f lower estimated mass-loss rates. However, these new mass-loss rates have not yet been x Application to the very low metallicity galaxy, I Zw 18-NW central He III region (Fig. 4), but does not enlarge the HII region or significantly heat the incorporated in any published grid of evolutionary models. 40 neutral gas. At Lx=1x10 erg/s (high state), howerver, the size of the He III sphere nearly UV spectral synthesis models of stellar populations at log Z/Z⊙ = -1.7 derived from SYNSPEC doubles, and even the outer H I region is quite warm. spectra (Lanz & Hubeny 03, 07) suggest an age of ~5 Myr for the I Zw 18-NW cluster. At this Figure 5. CLOUDSPEC model optical spectrum age, Geneva isochrones indicate that the maximum temperature is 40,000 K (at the MS turnoff)

and that all stars more massive than Mi=42 M⊙ have already completed their stellar lives. The spectra of OB stars, which dominate the UV flux of I Zw 18-NW show no sign of a stellar wind, and there is no evidence for a galaxy outflow.

Chandra observations in 2000 show that I Zw 18 harbors an ultra-luminous X-ray source, likely a massive X-ray binary (MXRB) composed of a black hole and stellar companion (Thuan+04). The 39 X-ray of I Zw 18 was Lx=3.2x10 erg/s. Later, XMM observations in 2002 caught I Zw 40 18 in a high state with Lx=1x10 erg/s! Kaaret & Feng+13 find that “the compact object is likely a near maximally with an unusually high mass, MBH > 85 M⊙. This is close to the maximum mass of 75 M⊙ predicted for black holes formed via stellar collapse in a low metallicity (Z/Z⊙ = 0. 019) environment (Belczynski+10).” The location of the MXRB in I Zw 18-NW is consistent with the center of a bright, compact [O III] source as well as the source of He II nebular emission.

Feedback from Massive Stellar Black Holes

We have used CLOUDSPEC (Hubeny & Heap 2000+) to model the spectra of very low-metallicity Figure 1: Initial Mass – Final Mass relation for very low metallicity galaxies with ultraluminous black holes. CLOUDSPEC uses CLOUDY (Ferland+13) to compute the and solar metallicity based on Geneva evolutionary models, circa ionization structure and emission-line spectra, and SYNSPEC (Lanz & Hubeny 95) to compute the 1992 – 2001. NLTE level populations and absorption-line spectra of the ionized and neutral gas. The models 6 39 shown below assume stellar parameters, Ṃ=1.x10 M⊙, age= 5 Myr, log Z/Z⊙=-2, BH Lx=1x10 6 o -3 erg/s, accretion disk temperature 1x10 K, interstellar density ne=10 cm , column density of 21 -2 Figure 4. CLOUDY Temperature profiles outer neutral region NH=2x10 cm , covering fraction 0.4, and no dust, similar to the parameters for I Zw 18-NW, but not attempting a fit. Nevertheless, our model spectra make a qualitative fit to the observed spectrum of I Zw 18-NW. Figure 6. CLOUDSPEC model spectrum

Top View