Jean-Rene Roy

Holmberg II - A laboratory for studying the violent interstellar medium

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society [Full text of this article is not available in the UHRA]

The Ionized Gas in the Aftermath of a Starburst: The Case of NGC 1569

Results from multislit optical spectroscopy of 16 H II regions and deep Hα imaging of the amorphous galaxy NGC 1569 are presented. The extinction across the main body of the galaxy, as derived from the Balmer Hα/Hβ line ratio, indicates that most of the observed extinction is taking place in our own Galaxy; the latter amounts to (AV)local = 1.61 ± 0.09, while the extinction due to NGC 1569 is (AV)intrinsic = 0.65 ± 0.04. The electron temperature was measured in three H II regions using the [O III] λ4363 line. The O/H distribution shows no gradient along the main axis of the galaxy, which is consistent with the behavior observed in other low-mass galaxies. The average metal abundance is 12 + log O/H = 8.26, with little scatter, suggesting, on one hand, that mixing mechanisms are very efficient throughout the main body of the galaxy, or, on the other, that the most recent nucleosynthetic products are hiding in a hot coronal gas phase. Up to 20% ± 4% of the global Hα emission originates from the faint diffuse halo component surrounding the main body of NGC 1569. We explore the possibility that runaway OB stars that are due to cluster ejection and associated with the burst of supernovae which ended ~5 × 106 yr ago could be responsible for most of the ionization of the halo gas.

The abundance gradient of NGC1365: evidence for a recently formed bar in an archetypal barred spiral galaxy?

Emission-line optical spectrophotometry for 55 H ii regions in the prominent southern barred spiral galaxy NGC 1365 is presented. Nebular diagnostic diagrams such as (N ii)/(O ii) and (S ii)/(O ii) versus ((O ii) + (O iii))/Hshow that the H ii regions of the barred galaxy have the same range of physical conditions as found in non-barred late-type galaxies. Extinction is moderately high across the disc and there is evidence for a slight trend of extinction with galactocentric distance; the logarithmic extinction at Hfalls from about c(H�) = 1.2 in the centre to 0.6 - 0.8 in the outer regions. The global O/H distribution has a moderate gradient of � 0.5 dex �0 1 (� 0.02 dex kpc 1 ) consistent with the known trend between the slope of the abundance gradient and the strength of the bar. A break is seen in the O/H gradient just beyond the -4/1 resonance, the gradient being moderately steep at � 0.8 dex � 1 0 (-0.05 dex/kpc) inside this resonance, and flat beyond �/�0 � 0.55. The abundance distribution is compared with another barred spiral galaxy, NGC 3359, and with that of two well- sampled normal spiral galaxies, NGC 2997 and M 101. The possibility that the bar formed recently in NGC 1365 is considered. The difficulties encountered in doing spectrophotometry with fibre optics are discussed and shown not to be insurmountable.

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.

The origin of broad emission lines in the extragalactic giant H II region NGC 2363

High signal-to-noise long-slit spectra have been obtained of the giant H II region NGC 2363 located in the dwarf SBm galaxy NGC 2366. A discovery of low-intensity broad spectral components (FWHM is approximately equal to 40 A or 2400 km/s) in the bright nebular lines H-alpha, H-beta, and forbidden O III is reported. The broad spectral components are detected over a large spatial extent (not less than 500 pc) centered on the nebula. Several mechanisms for broadening nebular lines are explored: stellar winds, Thomson scattering by hot gas, supernova remnants, and superbubble blowout. All mechanisms have problems. Superbubble blowout, which is the only known mechanism capable of accelerating interstellar gas over such a volume of space, does not appear consistent with the physical properties of the H II region NGC 2363 or with the nature of the host galaxy. It is concluded that the broad nebular lines are probably due to very high velocity gas whose origin is, at present, unknown.

The O/H distribution in the transition Magellanic galaxy NGC 1313

Multi-fibre emission-line spectrophotometry of 33 H II regions and 3 diffuse interstellar medium positions are presented for the barred Magellanic galaxy NGC 1313. The H II regions show a fairly narrow range of thermal conditions characteristic of high excitation nebular gas. Electron temperature was directly determined in four of the H II regions. The global O/H abundance distribution appears very flat across the disk at 12 + log O/H = 8.4 +/- 0.1, with possibly the bar regions having abundances higher by 0.2 dex than the outer disc. NGC 1313 is the highest mass barred galaxy known not to have any radial abundance gradient. The key role of the bar on the abundance distribution in disc galaxies is revisited.

The Ionizing Star Clusters of Giant H II Regions in NGC 2403

We present the results of a study on the massive star population down to about MV ~ -3.1, or 12–15 M⊙, of the most luminous giant H II regions in the nearby spiral galaxy NGC 2403, based on Hubble Space Telescope images and ground-based spectrograms. Particular emphasis is placed on the distribution of the Wolf-Rayet and red supergiant stars and the information they provide about the recent star-forming history of these large complexes. We find direct evidence for the presence of Wolf-Rayet (WR) stars in five of the six giant H II regions investigated; 25–40 WR stars are inferred for the sole NGC 2403-I giant H II region. Red supergiant (RSG) stars are mainly distributed over a more extended halo, while the young blue stars and most WR stars are in or close to a compact core. One appears to be seeing young cores of O and WR stars surrounded by older halos containing red supergiants. We propose a scenario in which RSG stars belonging to an early phase of star formation were followed by a more recent burst corresponding to a very blue mean sequence. Delayed trigger with preheating over several 100 pc by the first generation of massive stars allowed the build-up of the required confinement for the production of parsec-scale cluster cores with luminosity up to a few times 106 L⊙. Finally, we present some interesting objects found in the field of NGC 2403 outside the giant H II regions, such as field WR stars, globular clusters and background galaxies.

Nitrogen abundances in the amorphous galaxy NGC 5253

The central complex of ionized gas in the amorphous galaxy NGC 5253 was scanned with the slit of the RGO spectrograph using the imaging spectroscopy system of the Anglo-Australian Telescope. Monochromatic images corresponding to several nebular lines were obtained with a spatial resolution of 2.3 x 1.3 sq arcsec (1 pixel). The very high signal-to-noise ratio of the spectra allowed the measurements of the electron temperature over 78 pixels using the line ratio forbidden O III (4959+5007)/4363 and hence abundances of O, N, He, and Ne. A region of high values of log N/O was found to correspond with the presence of a cluster of Wolf-Rayet stars. Other areas of the gas complex show the normal deficiency in log N/O for this type of object, and no Wolf-Rayet feature is detected. There is an anticorrelation between log N/O versus log O/H, a trend which is not clearly consistent with any existing nucleosynthesis scenario for the origin of nitrogen in low metallicity galaxies. 41 references.

A search for Wolf-Rayet stars in active star forming regions of low mass galaxies: GR8, NGC 2366, IC 2574, and NGC 1569

We report the detection, via narrow-band λ4686 filter imagery, of possible new Wolf-Rayet stars in the most massive giant H II regions of the irregular galaxies NGC 2366 and IC 2574. One stellar knot in the post-starburst galaxy NGC 1569 also appears to contain a weak excess of light at λ4686. A similar search yielded negative results in the very low mass galaxy GR8. The strongest λ4686 excess is located close to the secondary eastern knot in the core of NGC 2366-I (NGC 2363). If this excess is of stellar origin, about five Wolf-Rayet stars of the luminous late-type (WNL) can account for the excess emission. Nebular emission wraps around this cluster in the form of a shell

Slow X-ray bursts and flares with filament disruption

The data from OGO-5 and OSO-7 X-ray experiments have been compared with optical data from six chromospheric flares with filament disruption associated with slow thermal X-ray bursts. Filament activation accompanied by a slight X-ray enhancement precedes the first evidence of Hα flare by a few minutes. Rapid increase of the soft X-ray flux accompanies the phase of fastest expansion of the filament. Plateau or slow decay phases in the X-ray flux are associated with slowing and termination of filament expansion. The soft X-ray flux increases as F∼(A + Bh) h, where h is the height of the disrupted prominence at any given time and A and B are constants. We suggest that the soft X-ray emission originates from a growing shell of roughly constant thickness of high-temperature plasma due to the compression of the coronal gas by the expanding prominence.