arXiv subjects

P. F. Roche

Publications and source records attributed to P. F. Roche.

42 records · Page 3Linked to original sources

arXiv2001

Infrared Spectroscopy of Substellar Objects in Orion

We present broad band spectra of a sample of 21 low luminosity sources in the Trapezium Cluster, with masses in the range 0.008 - 0.10 Msun (assuming an age of 1 Myr). These were selected for low extinction in most cases and are located west of the brighter nebulosity. The spectra are in the H bandpass (1.4-1.95 um) and K bandpass (1.9-2.5 um) also for most of the brighter sources, with a resolution of 50 nm. They were taken with UKIRT using the CGS4 spectrometer. Absorption by water vapour bands is detected in all the substellar candidates except one, which is a highly reddened object with strong H2 emission and an anomalously blue (I-J) colour, implying that it is a very young cluster member with circumstellar matter. The observation of prominent water vapour bands confirms the low Effective Temperatures implied by our (I-J) colour measurements in an earlier paper and would imply late M or L spectral types if these were older field dwarfs. However, the profiles of the H bandpass spectra are very different from those of field dwarfs with similar water absorption strength, demonstrating that they are not foreground or background objects. In addition, the CO absorption bands at 2.3 um and the NaI absorption feature at 2.21 um are very weak for such cool sources. All these features are quite well reproduced by the AMES-Dusty-1999 model atmospheres of Allard et al.(2000,2001), and arise from the low gravities predicted for the Trapezium sources. This represents a new proof of the substellar status of our sources, independent of the statistical arguments for low contamination, which are reexamined here. The very late spectral types of the planetary mass objects and very low mass brown dwarfs demonstrate that they are cluster members, since they are too luminous to be field dwarfs in the background.

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arXiv2001

8-13um Dust Emission Features in Galactic Bulge Planetary Nebulae

A sample of 25 IR-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8-13um spectroscopy. The classification of the warm-dust emission features provides a measure of the C/O chemical balance, and represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified dust types, 4 PNe have emission features associated with C-based grains, while the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe, <30%, contrasts with that for local PNe, around 80%, although it follows the trend for a decreasing frequency of C-rich PNe with galactocentric radius (paper~I). We investigate whether the PNe discussed here are linked to the bulge stellar population (similar to type~IV, or halo, PNe) or the inner Galactic disk (a young and super-metal-rich population). Although 60% of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfy our criteria, and they are probably linked to the inner Galactic disk. In the framework of single star evolution, the available information on bulge PNe points towards a progenitor population similar in age to that of local PNe (type I PNe are found in similar proportions), but super-metal-rich (to account for the scarcity of C-rich objects). Yet the metallicities of bulge PNe, as inferred from [O/H], fail to reach the required values - except for the C-rich objects. It is likely that the sample discussed here is derived from a mixed disk/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert (1992). The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a proportionally greater injection of silicate grains into the inner Galactic medium.

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arXiv2000

A High Resolution Radio Survey of Class I Protostars

We report the results of a survey of low mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formation were observed with the VLA at 0``.25 resolution. All seven sources drive CO outflows and display Herbig-Haro flows in the optical or near infrared wavebands. 4/7 sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, three sources were not detected to a 3-sigma limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3-sigma limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionisation fraction in the stellar wind. Temporal variability of the outflow may also play a role. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0``.4 scale at 2 cm and 3.5 cm. Follow-up imaging with MERLIN did not detect this source with a 0``.04 beam, indicating that the radio emission is generated in a region with a radius of about 25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of near infrared data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, which we originally detected through polarimetric imaging. We present a near infrared H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars.

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arXiv2000

The Galactic Disk Distribution of Planetary Nebulae With Warm Dust Emission Features: I

We investigate the galactic disk distribution of a sample of planetary nebulae characterised in terms of their mid-infrared spectral features. The total number of galactic disk PNe with 8-13mum spectra is brought up to 74 with the inclusion of 24 new objects, whose spectra we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample which we use to construct the distribution of the C/O chemical balance in galactic disk PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none, and SiC emission features coincide with a range of moderate N-enrichments. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth galactic quadrants. Two independent distance scales confirmed that the proportion of O-rich PNe decreases from 30+-9% inside the solar circle, to 14+-7% outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to C/O~1. They also have higher 6cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for warm-dust PNe. The requirement F(12um)>0.5Jy should probe a good portion of the galactic disk, and the dominant selection effects are rooted in the PN catalogues.

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arXiv2000

The Galactic Disk Distribution of Planetary Nebulae with Warm Dust Emission Features: II

Can the distribution of warm-dust compositions in IR-bright galactic disk PNe be linked to the underlying stellar population? The PNe with warm dust emission represent a homogeneous population, which is presumably young and minimally affected by a possible dependence of PN lifetime on progenitor mass. The sample in paper I thus allows testing the predictions of single star evolution, through a comparison with synthetic distributions and under the assumption that tip-of-the-AGB and PN statistics are similar. We construct a schematic model for AGB evolution (adapted from Groenewegen & de Jong 1993), whose free-parameters are calibrated with the luminosity function (LF) of C stars in the LMC, the initial-final mass relation, and the range of PN compositions. The observed metallicity gradient and distribution of star forming regions with galactocentric radius (Bronfman et al. 2000) allow us to synthesise the galactic disk PN progenitor population. We find the fraction of O-rich PNe, f(O), is a tight constraint on AGB parameters. For our best model, a minimum PN progenitor mass Mmin=1Msun predicts that about 50% of all young PNe should be O-rich, compared to an observed fraction of 22%; thus Mmin=1.2Msun, at a 2sigma confidence level. By contrast, current AGB models for single stars can account neither for the continuous range of N enrichment (Leisy & Dennefeld 1996), nor for the observation that the majority of very C-rich PNe have Peimbert type I (paper I). f(O) is thus an observable much easier to model. The decrease in f(O) with galactocentric radius, as reported in paper I, is a strong property of the synthetic distribution, independent of Mmin. This trend reflects the sensitivity of the surface temperature of AGB stars and of the core mass at the first thermal pulse to the galactic metallicity gradient.

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arXiv2000

A Population of very young Brown Dwarfs and free-floating Planets in Orion

We describe the results of a very deep imaging survey of the Trapezium Cluster in the IJH bands, using the UKIRT high resolution camera UFTI. Approximately 32% of the 515 point sources detected are brown dwarf candidates, including several free floating objects with masses below the Deuterium burning (planetary) threshold at 0.013 solar masses, which are detectable because of their extreme youth. We have confidence that almost all the sources detected are cluster members, since foreground contamination is minimal in the 33 arcmin^2 area surveyed and the dense backdrop of OMC-1 obscures all background stars at these wavelengths. Extinction is calculated from the (J-H)colours, permitting accurate luminosity estimates and temperatures are derived from the dereddened (I-J) colours. There is some evidence for a cut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the IMF. Since star formation is complete in the Trapezium this limit could have wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a process whose timescale will vary between star formation regions.

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