arXiv subjects

P. G. Martin

Publications and source records attributed to P. G. Martin.

At least 109 records · Page 6Linked to original sources

arXiv2004

On the O II ground configuration energy levels

The most accurate way to measure the energy levels for the O II 2p^3 ground configuration has been from the forbidden lines in planetary nebulae. We present an analysis of modern planetary nebula data that nicely constrain the splitting within the ^2D term and the separation of this term from the ground ^4S_{3/2} level. We extend this method to H II regions using high-resolution spectroscopy of the Orion nebula, covering all six visible transitions within the ground configuration. These data confirm the splitting of the ^2D term while additionally constraining the splitting of the ^2P term. The energies of the ^2P and ^2D terms relative to the ground (^4S) term are constrained by requiring that all six lines give the same radial velocity, consistent with independent limits placed on the motion of the O+ gas and the planetary nebula data.

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arXiv2004

Grain size distributions and photo-electric heating in ionized media

Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. A detailed code is necessary to model grains in a photo-ionized medium. In this paper we will use the spectral synthesis code Cloudy for this purpose. The grain model of this code has recently undergone a comprehensive upgrade. Part of this is the newly developed hybrid grain charge model, which will be described in detail. This model allows discrete charge states of very small grains to be modelled accurately while simultaneously avoiding the overhead of fully resolving the charge distribution of large grains, thus making the model both accurate and computationally efficient. A comprehensive comparison with the fully resolved charge state models of Weingartner & Draine (2001) shows that the agreement is very satisfactory for realistic size distributions. The effect of the grain size distribution on the line emission from photo-ionized regions is studied by taking standard models for an H II region and a planetary nebula and adding a dust component to the models with varying grain size distributions. A comparison of the models shows that varying the size distribution has a dramatic effect on the emitted spectrum, and affects the ionization balance as well. These results clearly demonstrate that the grain size distribution is an important parameter in photo-ionization models.

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arXiv2004

GSH23.0-0.7+117, a neutral hydrogen shell in the inner Galaxy

GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the VLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was detected. The expansion velocity and systemic velocity were determined through the systematic behavior of the HI emission with velocity. The center of the shell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is 6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume of the half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the medium in which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr, with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} erg can be provided by the stellar wind of a single O4 to O8 star over the age of the shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density (S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuum luminosity generated inside the shell. This upper limit implies a maximum of one O9 star (O8 to O9.5 taking into account the error in the distance) inside the HI shell, unless most of the incident ionizing flux leaks through the HI shell. To allow this, the shell should be fragmented on scales smaller than the beam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble has burst (as suggested by the HI channel maps), agreement between the energy and ionization requirements is even less likely. The limit set by the non-detection in the continuum provides a significant challenge for the interpretation of GSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable to other Galactic HI shells that have not been detected in the continuum.

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arXiv2003

Grain Size Distributions and Photo-Electric Heating in Ionized Regions

In this paper we present results obtained with the new grain code in Cloudy which underline the strong effect of photo-electric heating by grains in photo-ionized regions. We study the effect that the distribution of grain sizes has on the magnitude of the effect, and show that this effect is nothing short of dramatic. This makes the grain size distribution an important parameter in modeling of photo-ionized regions such as H II regions and planetary nebulae.

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arXiv2002

Temperature Variations from HST Spectroscopy of the Orion Nebula

We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t_A^2) across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate T_e and t_A^2 in the N^+ region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5" square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t_A^2 varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 K to 10232 K; t_A^2 varies from 0.00584 to 0.0175. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II] T_e exceeds the average [O III] T_e confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by theta^1 Ori C; and indirectly, evidence of high electron density.

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arXiv2002

Temperature Variations from HST Imagery and Spectroscopy of NGC 7009

We present new HST/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t_A^2) across the nebula. The WFPC2 T_e map is rather uniform; almost all values are between 9000 - 11,000 K, with the higher T_e's closely coinciding with the inner He^++-zone. The results indicate very small values - <~ 0.01 - for t_A^2 throughout. Our STIS data allow an even more direct determination of T_e and t_A^2, albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5'' square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10,139 K; t_A^2 is 0.0035. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot completely rule out temperature fluctuations along the line of sight as the cause of the large abundance discrepancy between heavy element abundances inferred from collisionally excited emission lines compared to those derived from recombination lines.

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arXiv2001

Grains in Photo-Ionized Environments

Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. This includes the diffuse ISM, as well as H II regions, planetary nebulae, and photo-dissociation regions. A detailed code is necessary to model grains in a photo-ionized medium since the interactions of grains with their environment include a host of microphysical processes, and their importance can only be judged by performing a complete simulation. In this paper we will use the spectral synthesis code Cloudy for this purpose. A comprehensive upgrade of the grain model has been recently incorporated in Cloudy, and certain aspects of this upgrade will be discussed. Special emphasis will be on the new grain charge model. We will consider in detail the physics of grains in both ionized and neutral environments, and will present a calculation of photo-electric heating and collisional cooling rates for a range of physical conditions and grain materials and for a range of grain sizes (including a realistic size distribution). We conclude with a brief discussion of the problems currently hampering progress in this field. The new grain model will be used to model the silicate emission in the Ney-Allen nebula, and will help us better understand the nature of the grains in that part of the Orion complex.

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arXiv2001

A Submillimeter View of Star Formation Near the HII Region KR 140

We present the results of 450 and 850 micron continuum mapping of the HII region KR 140 using the SCUBA instrument on the JCMT. KR 140 is a small (5.7 pc diameter) HII region at a distance of 2.3+/-0.3 kpc. Five of the six IRAS point sources near KR 140 were mapped in this study. Our analysis shows that two of these IRAS sources are embedded late B type stars lying well outside the HII region, two are a part of the dust shell surrounding the HII region, and one is the combined emission from an ensemble of smaller sources unresolved by IRAS. We have discovered a number of relatively cold submillimeter sources not visible in the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from 0.5 to 130 Msun. The distribution of masses for all sources is well characterized by a power law N(>M) ~ M^{-alpha} with alpha = 0.5 +/- 0.04, in agreement with the typical mass function for clumped structures of this scale in molecular clouds. Several of the submillimeter sources are found at the HII-molecular gas interface and have probably been formed as the result of the expansion of the HII region. Many of the submillimeter sources we detect are gravitationally bound and most of these follow a mass-size relationship expected for objects in virial equilibrium with non-thermal pressure support. Upon the loss of non-thermal support they could be sites of star formation. Along with the two B stars that we have identified as possible cluster members along with VES 735, we argue that five nearby highly-reddened stars are in a pre-main-sequence stage of evolution.

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arXiv2001

A Fast Technique for the Creation of Large-Scale High-Resolution IRAS (HIRES) Beam-matched Images

HIRES processing provides a significant improvement in both resolution and image quality over previous IRAS image products, but the characteristics of the HIRES beam make accurate comparisons between the various IRAS bandpasses and between HIRES data and data at other wavelengths non-trivial. We present a new, fast technique for the construction of HIRES beam-matched images that is especially well suited for the creation of large-scale (several square degrees) ratio maps. Other techniques for the construction of ratio maps are discussed and compared with the new algorithm. Examples of the large-scale ratio maps that can be constructed using this new technique are presented. The algorithm's application to the construction of multiwavelength difference images and multi-colour images is also demonstrated.

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arXiv2000

The HII Region KR 140: Spontaneous Formation of a High Mass Star

We have used a multiwavelength data set from the Canadian Galactic Plane Survey (CGPS) to study the Galactic HII region KR 140, both on the scale of the nebula itself and in the context of the star forming activity in the nearby W3/W4/W5 complex of molecular clouds and HII regions. From both radio and infrared data we have found a covering factor of about 0.5 for KR 140 and we interpret the nebula as a bowl-shaped region viewed close to face on. Extinction measurements place the region on the near side of its parent molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735, which is less than a few million years old. CO data show that VES 735 has disrupted much of the original molecular cloud for which the estimated mass and density are about 5000 $M_{\odot}$ and 100 cm$^{-3}$, respectively. KR 140 is isolated from the nearest star forming activity, in W3. Our data suggest that KR 140 is an example of spontaneous (i.e., non-triggered) formation of, unusually, a high mass star.

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arXiv2000

The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation

We present results from photoionization models of low-metallicity HII regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be non-negligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an HII region has an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that, regardless of its metallicity, it will have a negligibly small ICF. A similar, but metallicity dependent, result was found using the [O III] 5007/H$β$ ratio. These two results can be used as selection criteria to remove nebulae with potentially non-negligible ICFs. Using our metallicity independent criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of the rms scatter about the best fit $Y-Z$ line. A fit to the selected data results in a slight increase of the value of the primordial helium abundance.

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arXiv2000

Current Development of the Photo-Ionization Code Cloudy

This paper describes the most important differences between Cloudy 90 and Cloudy 94, the most recent upgrade of the code which has now been posted on the WWW. It also describes the upgrade of the grain model in Cloudy which is currently underway.

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arXiv1999

Properties of Dust Grains in Planetary Nebulae -- I. The Ionized Region of NGC 6445

In this paper we study new infrared spectra of the evolved planetary nebula NGC 6445 obtained with ISO. These data show that the thermal emission from the grains is very cool and has a low flux compared to H beta. A model of the ionized region is constructed, using the photo-ionization code CLOUDY 90.05. Based on this model, we show from depletions in the gas phase elements that little grain destruction can have occurred in the ionized region of NGC 6445. We also argue that dust-gas separation in the nebula is not plausible. The most likely conclusion is that grains are residing inside the ionized region of NGC 6445 and that the low temperature and flux of the grain emission are caused by the low luminosity of the central star and the low optical depth of the grains. This implies that the bulk of the silicon-bearing grains in this nebula were able to survive exposure to hard UV photons for at least several thousands of years, contradicting previously published results. A comparison between optical and infrared diagnostic line ratios gives a marginal indication for the presence of a t^2-effect in the nebula. However, the evidence is not convincing and other explanations for the discrepancy are also plausible. The off-source spectrum taken with ISO-LWS clearly shows the presence of a warm cirrus component with a temperature of 24 K as well as a very cold component with a temperature of 7 K. Since our observation encompasses only a small region of the sky, it is not clear how extended the 7 K component is and whether it contributed significantly to the FIRAS spectrum taken by COBE. Because our line of sight is in the galactic plane, the very cold component could be a starless core.

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arXiv1999

Emission-line Helium Abundances in Highly Obscured Nebulae

This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36$\micron$/[NeII] 12.8$\micron$, [NeIII]~15.6$\micron$ /[NeII] 12.8$\micron$, [ArIII] 9$\micron$/[ArII] 6.9$\micron$, and [ArIII] 21$\micron$/[ArII] 6.9$\micron$, that are sensitive to the He ICF. This happens because the ions cover a wide range of ionization, the line pairs are not sensitive to electron temperature, they have similar critical densities, and are formed within the He$^+$/H$^+$ region of the nebula. We compute a very wide range of photoionization models appropriate for galactic HII regions. The models cover a wide range of densities, ionization parameters, stellar temperatures, and use continua from four very different stellar atmospheres. The results show that each line pair has a critical intensity ratio above which the He ICF is always small. Below these values the ICF depends very strongly on details of the models for three of the ratios, and so other information would be needed to determine the helium abundance. The [Ar III] 9$\micron$/[ArII] 6.9$\micron$ ratio can indicate the ICF directly due to the near exact match in the critical densities of the two lines. Finally, continua predicted by the latest generation of stellar atmospheres are sufficiently hard that they routinely produce significantly negative ICFs.

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arXiv1999

A Mid-Infrared Galaxy Atlas (MIGA)

A mid-infrared atlas of part of the Galactic plane ($75^\circ < l < 148^\circ, b = \pm6^\circ$) has been constructed using HIRES processed infrared data to provide a mid-infrared data set for the Canadian Galactic Plane Survey (CGPS). The addition of this data set to the CGPS will enable the study of the emission from the smallest components of interstellar dust at an angular resolution comparable to that of the radio, millimetre, and far-infrared data in the CGPS. The Mid-Infrared Galaxy Atlas (MIGA) is a mid-infrared (12 $μ$m and 25 $μ$m) counterpart to the far-infrared IRAS Galaxy Atlas (IGA), and consists of resolution enhanced ($\sim 0.5'$ resolution) HIRES images along with ancillary maps. This paper describes the processing and characteristics of the atlas, the cross-beam simulation technique used to obtain high-resolution ratio maps, and future plans to extend both the IGA and MIGA.

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arXiv1999

A Mid-Infrared HIRES Atlas of the Galactic Plane

A mid-infrared atlas of the Galactic Plane (MIGA) has been constructed to provide a mid-infrared data set for the Canadian Galactic Plane Survey, an international project to map all of the major components of the interstellar medium of the Milky Way at a common resolution of approximately 1'. The atlas is based on HIRES processing of IRAS 12 and 25 micron data. Details of the MIGA production and applications of the atlas are discussed and illustrated.

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arXiv1999

Multiwavelength Studies of Interstellar Dust Using WIRE and MIGA

Studies of interstellar dust using the recently completed Mid-Infrared Galaxy Atlas (MIGA) and the soon to be launched Wide-Field Infrared Explorer (WIRE) satellite are summarized. Projects include looking at the distribution of dust around HII regions, the study of dust at the interfaces of HII regions and molecular clouds, and understanding the colour variations seen in cirrus clouds.

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arXiv1999

Classification of O Stars in the Yellow-Green: The Exciting Star VES 735

Acquiring data for spectral classification of heavily reddened stars using traditional criteria in the blue-violet region of the spectrum can be prohibitively time consuming using small to medium sized telescopes. One such star is the Vatican Observatory emission-line star VES 735, which we have found excites the HII region KR140. In order to classify VES 735, we have constructed an atlas of stellar spectra of O stars in the yellow-green (4800 - 5420 A). We calibrate spectral type versus the line ratio HeI 4922:HeII 5411, showing that this ratio should be useful for the classification of heavily reddened O stars associated with HII regions. Application to VES 735 shows that the spectral type is O8.5. The absolute magnitude suggests luminosity class V. Comparison of the rateof emission of ionizing photons and the bolometric luminosity of VES 735, inferred from radio and infrared measurements of the KR 140 region, to recent stellar models gives consistent evidence for a main sequence star of mass 25 M_sun and age less than a few million years with a covering factor 0.4 - 0.5 by the nebular material. Spectra taken in the red (6500 - 6700 A) show that the stellar H-alpha emission is double-peaked about the systemic velocity and slightly variable. H-beta is in absorption, so that the emission-line classification is ``(e)''. However, unlike the case of the more well known O(e) star Zeta Oph, the emission from VES 735 appears to be long-lived rather than episodic.

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