Engineering topics

Garnett, Donald R.

Publications and source records attributed to Garnett, Donald R..

7 records

NASA NTRS1994

Spatially resolved optical and near-infrared spectroscopy of the low-metallicity galaxy UGC 4483

UGC 4483 is a dwarf irregular galaxy in the M81 group. Narrow-band imaging has revealed an H II region in UGC 4483 with an H alpha flux approximately = 1 x 10 (exp - 13) ergs/sq cm/s. Optical and near-infrared spectroscopy of this H ll region yields He, N, O, Ne, and S abundances for the interstellar matter (ISM) in this galaxy. The spectra were acquired with several different telescope/instrument combinations in order to assess the quality of the derived uncertainties. With oxygen abundance of 3.3 x 10 (exp -5) (12 + log (O/H) = 7.5), this galaxy is similar to GR 8 and among the most metal poor dwarf irregulars known to date. However, the H II region in UGC 4483 has high excitation and higher surface brightness than GR 8, allowing very accurate abundance estimates. The N/O ratio is 3%, in good agreement with other low-metallicity dwarf galaxies. The S/O abundance ratio in UGC 4483 is close to the solar ratio, consistent with results for I Zw 18 and other low-metallicity dwarf irregulars. A He/H abundance of 0.079 +/- 0.002 is derived from observations of the lambda 6678 He ll emission line. A comparison of observations obtained with different telescopes and with the same telescope on different nights supports our relatively small estimated uncertainty in this measurement. We determine that the neutral helium fraction is insignificant through both direct observations of the He(+)/H(+) ratio across the nebula and through photoionization modeling. This results in a helium mass fraction of 0.239 +/- 0.006, consistent with the most recent theoretical and observational determinations of the primordial helium abundance. We also discuss remaining systematic uncertainties in the calculation of the primordial helium abundance

Skillman, Evan D.↗

NASA NTRS1994

A new ejecta shell surrounding a Wolf-Rayet star in the LMC

We have obtained CCD spectra of newly discovered shell-like nebulae around the WN4 star Breysacher 13 and the WN1 star Breysacher 2 in the Large Magellanic Cloud (LMC). The shell around Br 13 shows definite signs of enrichment in both nitrogen and helium, having much stronger (N II) and He I emission lines than are seen in typical LMC H II regions. From the measured electron temperature of about 17,000 K in the shell, we derive He/H and N/O abundance ratios which are factors of 2 and more than 10 higher, respectively, than the average LMC interstellar values. The derived oxygen abundance in the Br 13 shell is down by a factor of 8 compared to the local LMC interstellar medium (ISM); however, the derived electron temperature is affected by the presence of an incomplete shock arising from the interaction of the stellar wind with photoionized material. This uncertainty does not affect the basic conclusion that the Br 13 shell is enriched by processed material from the Wolf-Rayet star. In contrast, the shell around Br 2 shows no clear evidence of enrichment. The nebular spectrum is characterized by extremely strong (O III) and He II emission and very weak (N II). We derive normal He, O, and N abundances from our spectrum. This object therefore appears to be simply a wind-blown structure associated with a relatively dense cloud near the Wolf-Rayet star, although the very high-ionization state of the gas is unusual for a nebula associated with a Wolf-Rayet star.

Garnett, Donald R.↗

NASA NTRS1994

A very metal poor H II region in the outer disk of M101

We present new spectroscopic observations of an H II region in the extreme outer disk of the spiral galaxy M101, more than 32 kpc from the nucleus, or 25% farther out than the well-studied giant H II region NGC 5471. From a derived (O III) electron temperature of 13,900 K, we derive log O/H = -4.1, only 10% of the solar value, and smaller than the abundance measured in NGC 5471. Log N/O = -1.49, similar to the values seen in metal-poor dwarf galaxies, while log S/O = -1.74, essentially identical to the solar value, confirming the trend of constant S/O observed in earlier studies. With the inclusion of this new object, the composition gradient in M101 from published spectroscopic observations shows no evidence for either a break in the gradient from 4 to 33 kpc, or a rise in the abundances in the outer parts of the disk.

Garnett, Donald R.↗

NASA NTRS1993

H II regions as probes of galaxy evolution and the properties of massive stars

The use of H II regions as probes to study the chemical evolution of galaxies and the spectral properties of hot, massive stars is reviewed. The observable parameters for this task are the physical conditions, elemental abundances, and ionization balance in the ionized gas. Some outstanding uncertainties in the determination of these parameters and some approaches to remedy or circumvent the problems are discussed.

Garnett, Donald R.↗

NASA NTRS1989

Utraviolet observations of the planetary nebula NGC 2242

IUE observations of NGC 2242 reveal strong emission lines of C IV, He II, semi-forbidden C III, and forbidden Ne IV, along with weaker emissions from O IV and N IV. It is found that the oxygen abundance is about 1/6 of the solar value for T(e) = 25,000 K, and that the N/H and Ar/H ratios are down by a factor of 2-3 from typical disk PN values. The present abundance pattern resembles that observed in the halo planetary DDDM-1, except for the unusually low C/O ratio in DDDM-1.

Garnett, Donald R.↗

NASA NTRS1988

The abundances of carbon and nitrogen in I Zw 18

The first measurements of the semiforbidden C III 1909 A and forbidden N II 6583 A emission lines in I Zw 18 are reported. These data are combined with previously published spectrophotometry in order to derive the carbon and nitrogen abundances in I Zw 18 with the aid of a nebular model. The CNO abundances in I Zw 18 are found to exceed those found in the Magellanic Clouds and other well-observed oxygen-poor irregular galaxies and are more similar to those in the sun and nearby Galactic H II regions. It is argued that I Zw 18 may be older and more chemically evolved than previously believed, and that its extremely low O/H results from escape of O-rich supernova ejecta from the Galaxy or from dilution by accretion of primordial gas. Alternatively, I Zw 18 may have formed from a preenriched intergalactic medium. Spectroscopic diagnostics for composite photoionization models to fit the spectrum of I Zw 18 are discussed.

Dufour, Reginald J.↗

NASA NTRS1988

Spectroscopic observations of the high-ionization planetary nebula NGC 2242 and the H II region K4-45

New optical spectra of two suspected planetary nebulae, K4-45 and NGC 2242, are presented. It is shown that K4-45 (PK 96 + 1) is a reddened H II region with approximately solar abundances, excited by an early B type star. NGC 2242 (PK 170 + 15) is a planetary nebula of exceptionally high ionization, exhibiting He II, and forbidden Ne V, Ar V, Ne III, and O III emission lines. The lack of He I emission and the presence of strong forbidden Ne V and He II emission indicate an optically thin nebula that is ionized by a central star with an effective temperature greater than 100,000 K.

Garnett, Donald R.↗