Carmelle Robert

Starburst99: Synthesis Models for Galaxies with Active Star Formation

Starburst99 is a comprehensive set of model predictions for spectrophotometric and related properties of galaxies with active star formation. The models are an improved and extended version of the data set previously published by Leitherer & Heckman. We have upgraded our code by implementing the latest set of stellar evolution models of the Geneva group and the model atmosphere grid compiled by Lejeune et al. Several predictions which were not included in the previous publication are shown here for the first time. The models are presented in a homogeneous way for five metallicities between Z = 0.040 and 0.001 and three choices of the initial mass function. The age coverage is 106—109 yr. We also show the spectral energy distributions which are used to compute colors and other quantities. The full data set is available for retrieval at a Web site, which allows users to run specific models with nonstandard parameters as well. We also make the source code available to the community.

Starbursts and Star Clusters in the Ultraviolet

Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

Deposition of mass, momentum, and energy by massive stars into the interstellar medium

Models for the integrated output of mass, momentum, and energy from a population of massive stars are presented. We computed an extensive grid of radiatively wind models for hot stars and derived scaling relations for the mass-loss rates and wind velocities as a function of stellar parameters, including the metal abundance. It is found that the mass-loss rate scales with metal abundance like M∞Z 0.8 . These wind models are combined with empirical stellar-wind data of other stellar types to obtain predictions for mass-loss rates and wind velocities in the entire upper Hertzsprung-Russell diagram

Hubble Space Telescope discovery of candidate young globular clusters in the merger remnant NGC 7252

New, high-resolution images of the central region of NGC 7252 obtained with the Planetary Camera of the HST are presented. NGC 7252 is a prototypical example of a remnant of two merged disk galaxies. Our most striking result is the discovery of a population of about 40 blue pointlike objects in this galaxy. The mean absolute magnitude of these objects is Mv = -13 mag; the mean color is V-I = 0.7 mag; and the mean effective radius is 10 pc. The luminosities, colors, projected spatial distribution, and sizes are all compatible with the hypothesis that these objects formed within the last 1 Gyr following the collision of two spiral galaxies, and that they are young globular clusters. It therefore appears that the number of globular clusters may increase during the merger of gas-rich galaxies. This weakens van den Berghs objection against ellipticals being formed through disk mergers, based mainly on the fact that disk galaxies have fewer globular clusters per unit luminosity than ellipticals do. NGC 7252 shows a single, semistellar nucleus; relatively bright spiral structure is seen within 1.6 kpc of the center, presumably formed through the continued infall of gas into a disk around the center of the galaxy.

Spectroscopic evidence for rapid blob ejection in Wolf-Rayet stars

The five bright Cygnus WN stars in the unblended He II 5411A emission line has been monitored at high spectral resolution and high S/N during four or five nights. Typical data are presented here for the two single WN6 stars HD 191765 and HD 192163. The spectra reveal narrow emission bumps superposed on a broad smooth background wind profile with FWHM of about 2-10 A and height 10 percent or less in continuum units for HD 191765. The bumps always appear to accelerate outward along with the general wind, on a time scale of hours. 20 references.

Ultraviolet Line Spectra of Metal-poor Star-forming Galaxies

We present synthetic ultraviolet spectra of metal-poor star-forming galaxies that were calculated with the Starburst99 package. A new spectral library was generated from Hubble Space Telescope observations of O stars in the Large and Small Magellanic Clouds. The corresponding mean metallicity of the synthetic spectra is approximately The spectra have a resolution of 1 and cover the spectral 1 Z _ .A e range 1200¨1600 A set of model spectra was calculated for a standard initial mass function and star Ae . formation history and is compared to synthetic spectra at solar metallicity. We —nd that the spectral lines are generally weaker at lower metallicity, as expected from the lower elemental abundances. Stellar wind lines, however, show a more complex behavior: the metallicity dependence of the ionization balance can be important in trace ions, like N4‘ and Si3‘. Therefore, the strength of N V j1240 and Si IV j1400 does not scale monotonically with metallicity. We compare our new models to ultraviolet spectra of NGC 5253 and MS 1512-cB58, two star-forming galaxies with one-fourth solar metallicity at low and high redshifts, respectively. The new library provides signi—cantly better —ts to the observations than earlier models using the library. We discuss the potential of utilizing stellar photospheric and Z _ wind lines to estimate the chemical composition of star-forming galaxies. The new metal-poor synthetic spectra are available via the Starburst99 Web site.

Far-Ultraviolet Spectra of Starburst Galaxies: Stellar Population and the Kinematics of the Interstellar Medium*

The far-ultraviolet spectra of the four starburst galaxies NGC 6090, Mrk 66, Mrk 1267, and IRAS 0833+6517 were observed with the Hopkins Ultraviolet Telescope during the Astro-2 mission. Additional data were obtained for IRAS 0833+6517 with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope. We analyze the observations in terms of the stellar content and the kinematics of the interstellar medium, and we discuss the implications of these results for the interpretation of the ultraviolet spectra of high-redshift galaxies. Evolutionary synthesis models are used to constrain the star formation history from the absolute ultraviolet flux and from the stellar components of the absorption lines of Si IV and C IV, and from the far-ultraviolet lines O VI + Ly? + C II. The spectral energy distributions from these models are used as inputs for the photoionization code CLOUDY to predict [O III]/H? and column densities of the cool component of the ionized gas (Si II and C II) associated with the H II region. The results indicate continuous star formation during about 9 Myr, or, alternatively, bursts with ages between 3 and 6 Myr, and a mass consistent with that estimated from the H? flux. Evidence for significant dilution by a field star population is found in all galaxies except NGC 6090. Most of the interstellar absorption lines are saturated. Their equivalent widths indicate a large velocity dispersion in the gas. Other evidence for large-scale motions of the interstellar gas comes from blueshifts of several hundred km s-1 with respect to the systemic velocity in the interstellar lines of NGC 6090, Mrk 66, and IRAS 0833+6517. These outflows are most likely driven by the starburst. Ly? was detected in emission in three of the galaxies (NGC 6090, Mrk 66, and IRAS 0833+6517). The dereddened Ly?/H? ratio in IRAS 0833+6517 is close to the recombination value, indicating that extinction is more important than multiple resonant scattering effects. However, the GHRS spectrum of IRAS 0833+6517 clearly shows that the emission profile of Ly? is asymmetric, the blue wing being absorbed by neutral gas. This indicates that the velocity structure of the neutral gas and the scattering by H I atoms can also play an important role in the escape of the Ly? photons.

Physical Parameters of Erupting Luminous Blue Variables: NGC 2363-V1 Caught in the Act*

A quantitative study of the luminous blue variable NGC 2363-V1 in the Magellanic galaxy NGC 2366 (D = 3.44 Mpc) is presented, based on ultraviolet and optical Hubble Space Telescope STIS spectroscopy. Contemporary WFPC2 and William Herschel Telescope imaging reveals a modest V-band brightness increase of ~0.2 mag per year between 1996 January-1997 November, reaching V = 17.4 mag, corresponding to MV = -10.4 mag. Subsequently, V1 underwent a similar decrease in V-band brightness, together with a UV brightening of 0.35 mag from 1997 November to 1999 November. The optical spectrum of V1 is dominated by H emission lines, with Fe II, He I and Na I also detected. In the ultraviolet, a forest of Fe absorption features and numerous absorption lines typical of mid-B supergiants (such as Si II, Si III, Si IV, C III, C IV) are observed. From a spectral analysis with the non-LTE, line-blanketed code of Hillier & Miller, we derive stellar parameters of T* = 11 kK, R* = 420 R?, log (L/L?) = 6.35 during 1997 November, and T* = 13 kK, R* = 315 R?, log (L/L?) = 6.4 for 1999 July. The wind properties of V1 are also exceptional, with 4.4 ? 10-4M? yr-1 and v? 300 km s-1, allowing for a clumped wind (filling factor = 0.3) and assuming H/He ~ 4 by number. The presence of Fe lines in the UV and optical spectrum of V1 permits an estimate of the heavy elemental abundance of NGC 2363 from our spectral synthesis. Although some deficiencies remain, allowance for charge exchange reactions in our calculations supports a SMC-like metallicity, that has previously been determined for NGC 2363 from nebular oxygen diagnostics. Considering a variety of possible progenitor stars, V1 has definitely undergone a giant eruption, with a substantial increase in stellar luminosity, radius, and almost certainly mass-loss rate, such that its stellar radius increased at an average rate of ~4 km s-1 during 1992 October-1995 February. The stellar properties of V1 are compared to other LBVs, including ? Car and HD 5980 during its brief eruption in 1994 September, the latter newly analyzed here. The mass-loss rate of the HD 5980 eruptor compares closely with V1, but its bolometric luminosity was a factor ~6 times larger.

Synthetic UV Lines of SI iv, C iv, and He II from a Population of Massive Stars in Starburst Galaxies

We present the first results of our study of the massive star population in starburst galaxies based on UV data. We have synthesized the Si IV λ1400, C IV λ1550 and He II λ1640 lines for both a continuous and an instantaneous burst of star formation with approximately solar chemical composition. Our code uses the latest generation of stellar evolutionary models, stellar atmosphere codes, and a library of high-dispersion IUE spectra of hot stars. Models were computed for various values of the IMF parameters. Si IV λ1400 and C IV λ1550 develop P Cygni profiles when formed in strong stellar winds from the most massive stars

Observations of stellar winds from hot stars at 1.3 millimeters

The detection of seven hot stars at a wavelength of 1.3 mm with the Swedish-ESO Submillimeter Telescope at La Silla is reported. The measured millimeter fluxes are combined with existing centimeter data to study the radio spectrum. The spectral index of ∼0.6 is consistent with emission from optically thick free-free radiation in an isotropic, isothermal outflow expanding at constant velocity. Several mechanisms which can lead to spectral indices different from 0.6 are discussed.