The Chemical Evolution of Dynamically Hot Systems Michael Richer Observatorio Astronómico Nacional Instituto de Astronomía, UNAM, Mexico collaborators: Marshall L. McCall Grażyna Stasińska This is work I’m doing in collaboration with Marshall McCall at York University in Canada. photo: José Alberto López

The road may be difficult… Measuring the chemical abundances in extragalactic PNe is relatively straightforward. Interpreting those abundances to infer the chemical evolution of their host galaxies is not necessarily straightforward. It is often not easy to reconcile all of the available data into a coherent picture. Sometimes, too, it is necessary to extrapolate results based upon “reasonable” assumptions, but I realize that these may not necessarily appear reasonable to everyone. Undoubtedly, some of you will take exception to some of my conclusions. photo: José Manuel Murillo

Why are PNe of interest? In a star-forming galaxy the current ISM abundances may be determined from H II regions. Usually, the metallicity is characterized by the oxygen abundance. the past ISM abundances may be derived from stars and PNe, but the epoch corresponding to these abundances is uncertain. In a DHS (ellipticals, dwarf spheroidals, and the bulges of spirals) there is no star formation and often no ISM, so some probe besides H II regions will be necessary. it is still possible to measure abundances in stars and PNe, with the same uncertainties as above. it is often difficult to observe stars unless they are much brighter than the background or the background is uncrowded. abundances have traditionally been measured from integrated spectra and calibrated in terms of the iron abundance, e.g., using the Mg2 index. PNe are one of the few direct probes of chemical abundances in DHSs. NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Spectroscopy of extragalactic PNe Spectroscopy of extragalactic PNe is difficult due to the need/desire to detect the faint, temperature-sensitive line of [O III]l4363. He IIl4686 as well as low ionization lines can also be intrinsically faint, but this is generally more manageable. Subtracting the light of the background galaxy (often bright) can also be complicated because of its spatial structure and spectral properties. Good discusions of the observational challenges are given by Jacoby & Ciardullo 1999, ApJ, 515, 169 (multi-slit spectroscopy) Walsh et al. 1999, A&A, 346, 753 (long slit spectroscopy) Roth et al. 2004, ApJ, 603, 531 (integral field spectroscopy) Published data have come from 4m class telescopes. Observations with 8m telescopes should allow detection of [O III]l4363 in PNe out to at least 5 Mpc, farther if high dispersion (R ~ 10,000) or high spatial resolution are used to suppress the galaxy background.

Spectroscopy: reliability There is still little overlap between observational samples among different observers. On “simple” backgrounds, the line intensities are probably uncertain to within 25% for lines brighter than Hb and to within a factor of two for lines half the intensity of Hb. uncertainty in line fitting and continuum placement uncertainty in reddening uncertainty in atmospheric extinction, refraction, etc.

What chemical abundances can be studied in PNe? Nucleosynthesis in the stellar progenitors of all PNe modify their initial abundances of He, C, N, and s-process elements. Theoretical models of AGB stars are still incapable of producing low-mass carbon stars, M < 3M, so it is not clear how efficient the third dredge-up is for the lowest mass PN progenitors. It is clear that many PNe of all types are C-rich. Nucleosynthesis in the stellar progenitors of some PNe may modify their initial abundances of O and possibly Ne. Theoretical studies indicate that this affects PN progenitors of higher mass, typically, M > 3M. The extent and sign of the effect it is unclear. Except for the abundance of He, all abundances in extragalactic PNe are determined from collisionally-excited lines (forbidden lines). Note that chemical abundances may only be studied in the intrinsically brightest extragalactic PNe, i.e., it is not possible to study the entire population. photo: José Alberto López

Nucleosynthesis in PN progenitors The PN progenitors can modify N/O much more than He/H. In all galaxies, the N/O ratios in bight PNe cover the same range. Nucleosynthesis and dredge-up in the progenitors of bright PNe then appears to be the same everywhere. photo: José Alberto López

O/H and Ne/H in bright PNe In the ISM of star-forming galaxies, the production of O and Ne is believed to be dominated by type II SNe. Bright PNe in all galactic systems studied thus far follow the relation between O and Ne abundances observed in the ISM in star-forming galaxies. The simplest conclusion is that the stellar progenitors of bright PNe do not modify their original O and Ne abundances significantly, i.e., at the 0.3 dex level for these observations. photo: José Alberto López

Issues that affect the interpretation of chemical abundances in PNe SAMPLE SELECTION: often based upon high [O III]l5007 luminosity nucleosynthesis during the evolution of the PN progenitor stars the history of star formation higher stellar death rates from younger stellar populations the evolution time scale of post-AGB stars evolution time scale from AGB to PN phases absolute evolutionary time scales the initial mass-final mass relation mass loss during the AGB, post-AGB (and RGB?) phases relationship between nebular morphology and progenitor mass? All of the above may/should affect the population of the PNLF. If so, they could also affect the interpretation of PN abundances, since only the brightest PNe may usually be studied (usually “luminosity” = [O III]l5007 luminosity). NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Observations: systems with on-going star formation In the Magellanic Clouds, young stars and H II regions also have similar O/H (Hill et al. 1995, A&A, 293, 347). In NGC 6822, young stars, H II regions, and bright PNe have similar abundances (Venn et al. 2001). The stellar abundances are determined from recombination lines while the chemical abundances in the PNe and H II regions are determined from forbidden lines. In the LMC, the mean abundance found for a sample of PNe depends upon the luminosity range spanned by the sample (Richer 1993, ApJ, 415, 240). more luminous PNe  higher mean O/H. O/H in brightest PNe = O/H in H II regions. Venn et al. (2001):  = young stars,  = H II regions,  = PNe photo: José Alberto López

Observations: systems where star formation stopped “long ago” Milky Way bulge: McWilliam & Rich (1994) found a mean [Fe/H] of -0.25 dex and [O/Fe] > 0 for bulge giants. The bright PNe in the Milky Way bulge have a similar mean [O/H] (-0.26 dex; Stasinska et al. 1998) if the Anders & Grevesse (1989) solar abundance is used (-0.02 dex using the Allende-Prieto et al. 2000 scale). Fornax and Sagittarius dwarf spheroidals: Various recent studies find that O/H in the more metal-rich stars is similar to that found in the PNe (Anders & Grevesse 1989 scale). It is unlikely that their PNe arise from more metal-poor stellar populations, since the [O/Fe] ratios implied would then be extremely high (difficult to understand). These two galaxies are problem cases given their small masses and PN populations. Their PNe are probably also not intrinsically bright. Given the example of the Milky Way, it will therefore be assumed that the mean O/H in the PNe is the same as that in the stars. Suitable (downward) corrections will be made for Sagittarius and Fornax. This is the crucial assumption upon which the conclusions depend. photo: José Alberto López

Metallicity-luminosity relation for DHSs DHSs follow a metallicity-luminosity relation. The mean O/H in DHSs is higher than that observed in equally luminous dwarf irregulars. The maximum O/H in DHSs differs even more. This is also seen in the He abundances in the dwarf spheroidals (though not the bulges of M31 & Milky Way): at a given O/H, He/H is higher in the PNe in dwarf spheroidals than in their counterparts in dwarf irregulars. If they have evolved from dwarf irregulars like those observed today, dwarf spheroidals must have faded by at least ~4 mag, which is unlikely. DHSs appear to have incorporated their O production into stars more efficiently than today’s dwarf irregulars. photo: José Alberto López

Supernova-driven winds? The oxygen abundances in DHSs appear to correlate with the stellar velocity dispersions. Such a correlation is expected if supernova-driven winds are the agent that terminates chemical evolution in these systems. photo: José Alberto López

Gas outflows during star formation? In order to simultaneously fit the means and dispersions in O/H observed in DHSs with a model of chemical evolution that incorporates supernova-driven winds, it is necessary to allow gas outflow while the galaxies are forming stars. The large dispersions in O/H are another indication of efficient incorporation of oxygen production into stars. Gas outflow increases the efficiency of the incorporation of newly-synthesized elements because the ISM loses mass during the life span of the stars responsible for enriching the ISM. An efficient incorporation of newly-synthesized elements into stars need not imply an efficient conversion of matter into stars. McCall & Richer (2003, in IAU Symp 209, p. 583) photo: José Alberto López

Conclusions It appears to be feasible to use O/H in bright PNe as probes of the chemical evolution in DHSs. DHSs follow a metallicity-luminosity relation. Supernova-driven winds are a natural explanation for such a relation in galaxies that are now devoid of interstellar matter. The metallicity-luminosity relation for DHSs is displaced to higher O/H compared to that followed by dwarf irregulars. It is unlikely that this displacement is due to fading of the DHS population, but rather to a more efficient incorporation of their element production into new stars. Initial modelling of these results indicates that DHSs suffered gas outflows even while forming their stars. Making better use of PNe as probes of chemical evolution requires a better quantitative understanding of PN evolution. photo: José Alberto López

The history of star formation? Connecting PNe and stars In star-forming galaxies, the masses and ages of the different stellar populations matter. the younger stellar populations have higher death rates, so the mean O/H measured in bright PNe is biased towards the value observed in the ISM. In galaxies where star formation stopped “long ago” (> 1 Gyr), the masses of the different stellar populations matter. the difference between the mean O/H in bright PNe and the value that was achieved in the ISM grows as a function of the final O/H achieved in the ISM. Richer et al. (1997, A&AS, 122, 215) NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Evolution from AGB to PN The envelope mass of the central star when it evolves off the AGB has a dramatic impact upon the time scale for evolution from the AGB to PN phases. The number of bright central stars is a strong function of the envelope mass. The duration of the phase during which central stars are bright is also a function of the envelope mass. This issue may not matter for studies of chemical evolution if only galaxies with or without star formation are considered. MeR random; MeR < 0.1 M MeR = f(M) MeR constant; MeR < 5x10-3 M Mc = 0.569 M Mc = 0.535 M MeR random; MeR < 10-4 M Mc = 0.9 M Stanghellini & Renzini (2000, ApJ, 542, 308): : PN nuclei; : wind objects; : proto PN nuclei; : post PN nuclei NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

The time scale of post-AGB evolution? Simple models indicate that PNe should cluster in defined regions of these diagrams, but observed data do not. To fit the data for M31 and the LMC requires different evolution for both the nebular shells and the central stars. Is this a problem with the models, the time scale of post-AGB evolution, or something else? Observations of faint PNe should be able to solve this. Stasinska et al. (1998, A&A, 336, 667) NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

PN morphology At least in the Magellanic Clouds, there is a lot of mixing between [O III]l5007 luminosity and morphology this mixing is not so strong for Balmer lines In the Milky Way disk, PN morphology varies with scale height above the disk plane, so should be a function of progenitor mass. Morphology likely affects PN luminosity since it will affect the angular distribution of the optical depth. This issue may also not matter for studies of chemical evolution so long as only galaxies with or without star formation are considered. Stanghellini et al. (2003, ApJ, 596, 997): : round, : point-symmetric; : elliptical; : bipolar core; : bipolar or quadrupolar NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Connection between nebular and central star evolution? left: Villaver et al. (2002, ApJ, 581, 1204) right: Richer & López (unpublished) How long are PNe bright? Upon what progenitor properties does this depend? Expansion velocities are easily measured within the Local Group… Hydrodynamical models that account for the connection between the evolution of the central star and that of the nebular envelope indicate that the relation between the nebular kinematic age and the central star age is complicated. NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)