arXiv subjects

C. J. Davis

Publications and source records attributed to C. J. Davis.

83 records · Page 5Linked to original sources

arXiv2005

A mean redshift of 2.8 for Swift gamma-ray bursts

The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z_mean), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z_mean = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.

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arXiv2005

Near-Infrared echelle spectroscopy of Proto-Planetary Nebulae: probing the fast wind in H2

Echelle spectroscopy of H2, [FeII] and Br-gamma line emission from a very young planetary nebula, IRAS 21282+5050, and from four proto-planetary nebulae, IRAS 19343+2926 (M 1-92), IRAS 17150-3224 (AFGL 6815), IRAS 17423-1755 (Hen 3-1475), and IRAS 17441-2411 is presented. H2 line emission is detected in discrete shock fronts in the lobes of each nebula, regardless of source spectral type. In IRAS 17150-3224 we also uncover possible signs of rotation, as would be expected if the H2 features were excited in a magneto-centrifugal disk wind. [FeII] emission was detected in only one source, M 1-92. The H2 and [FeII] observations of M 1-92, and the complex H2 profiles in IRAS 21282+5050, are explained using the shock models of Smith and collaborators. J-type bow models are adequate in each case, i.e. a strongly-magnetised wind is not required. Finally, Br-gamma is detected in each of the five targets; in absorption in the G-type PPN though in emission in the O and B-type sources. Br-gamma emission is detected predominantly toward the near-IR continuum peak in each PPN. Low peak velocities, though very broad profile widths, are measured in each target. The emission must therefore derive from ionised regions in a fast wind very close to the central star, or, in the late-type sources, from absorption in an equatorial torus.

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arXiv2004

Embedded Star Clusters in the W51 Giant Molecular Cloud

We present sub-arcsecond (0.35"-0.9"), near-infrared J,H,K band photometric observations of six fields along the W51 Giant Molecular Cloud (W51 GMC). Our observations reveal four new, embedded clusters and provide a new high-resolution (0.35") view of the W51IRS2 (G49.5-0.4) region. The cluster associated with G48.9-0.3 is found to be a double cluster enclosed in a nest of near-infrared nebulosity. We construct stellar surface density maps for four major clusters in the W51 GMC. These unveil the underlying hierarchical structure. Color-color and color-magnitude diagrams for each of these clusters show clear differences in the embedded stellar populations and indicate the relative ages of these clusters. In particular, the clusters associated with the HII regions G48.9-0.3 and G49.0-0.3 are found to have a high fraction of YSOs and are therefore considered the youngest of all the near-infrared clusters in the W51 GMC. The estimated masses of the individual clusters, when summed, yield a total stellar mass of ~10^4 Msun in the W51 GMC, implying a star formation efficiency of 5-10%. These results in comparision with the CO observations of the W51 GMC, suggest for the first time, that star formation in the W51 GMC is likely triggered by a galactic spiral density wave.

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arXiv2004

A search for radioactive 26Al in the nova-like variable V4332 Sagittarii

We have searched for the important radioactive isotope 26Al in the nova-like source V4332 Sgr. Recent results from gamma ray astronomy show that there is pervasive emission of the 1.809 MeV gamma ray photon, arising from the decay of 26Al to 26Mg, from all over the galactic plane. Though the sites from where this emission originates are not clearly established, novae are believed to be an important contributing source. In this context, V4332 Sgr presented a rare opportunity to observationally investigate whether novae or novae-like sources synthesize 26Al and to what extent. Strong AlO bands in the near-IR have been reported in this object recently. As molecular bands of different isotopic compositions are readily resolved spectroscopically (e.g. 12CO and 13CO), it was thought that the components of AlO associated with 26Al and stable 27Al could be detected as separate bands. Our spectra indicate that there is no strong presence of 26Al in V4332 Sgr. A reliable upper limit of 0.10 for the 26Al/27Al ratio is determined which constitutes the first observational constraint for this ratio in a potential 26Al producing source. While V4332 Sgr is not a typical nova, its outburst amplitude and light-curve behaviour bear close similarity to that of novae. Hence, although the results from V4332 Sgr cannot be directly extended to novae in general, the limit on the observed 26Al/27Al ratio could be a useful input in constraining rather uncertain nucleosynthesis models for the production of 26Al in novae/novae-like sources. By comparing the observed 26Al/27Al ratio in V4332 Sgr with that expected in classical novae it appears unlikely that the progenitor of V4332 Sgr is an Oxygen-Neon-Magnesium white dwarf.

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arXiv2003

Paschen beta emission as a tracer of outflow activity from T-Tauri stars, as compared to Optical Forbidden emission

The Paschen beta (1.2822 micron) emission line found in the near-infrared spectra of T-Tauri stars (TTSs) is believed to trace the accretion of material onto the central star. We present spectroscopic data which suggests that this may not always be the case. The technique of spectro-astrometry is used by us to measure positional displacements in the Pab emission from four T-Tauri stars, namely DG Tau, V536 Aql, LkHalpha 321 and RW Aur. We also observed the optical forbidden emission from these sources. Forbidden emission lines are formed in the outflows that accompany the evolution of protostars and so are ideal to use as a comparison to confirm that the measured offsets in the Pab emission are indeed due to outflowing material. Models based on the magnetospheric accretion theory have been the most successful to date in explaining the origin of atomic hydrogen emission lines. Yet we see that the line profiles of the sources showing displacement in their Pab emission all have features that the magnetospheric accretion model has so far failed to explain, such as broad full width half maxima, large wings and an absence of red shifted absorption features. The failure of the models to explain the presence of large extended wings in the line profiles is particularly interesting in the context of this study as in all cases it is in the extended wings that we measure offsets in position with respect to the source.

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arXiv2003

A Ring Shaped Embedded Young Stellar (Proto)Cluster

We present sub-arcsec (FWHM ~ 0.5") J, H, K and L' images of a young stellar cluster associated with a candidate massive protostar IRAS22134+5834. The observations reveal a centrally symmetric, flattened cluster enclosing a central dark region. The central dark region is possibly a cavity within the flattened cluster. It is surrounded by a ring composed of 5 bright stars and the candidate massive protostar IRAS22134+5834. We construct JHKL' color-color and HK color-magnitude diagrams to identify the young stellar objects and estimate their spectral types. All the bright stars in the ring are found to have intrinsic infrared excess emission and are likely to be early to late B type stars. We estimate an average foreground extinction to the cluster of A_v \~ 5mag and individual extinctions to the bright stars in the range A_v ~ 20-40mag indicating possible cocoons surrounding each massive star. This ring of bright stars is devoid of any HII region. It is surrounded by an embedded cluster making this an example of a (proto)cluster that is in one of the dynamically least relaxed states. These observations are consistent with the recent non-axisymmetric calculations of Li & Nakamura, who present a star formation scenario in which a magnetically subcritical cloud fragments into multiple magnetically supercritical cores, leading to the formation of small stellar groups.

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arXiv2002

The impact of the Herbig Haro object HH2 on local dust and gas

We present results from a study of molecular gas and dust in the vicinity of the Herbig Haro object HH2. Emission from the sub-mm continuum, 12CO and HCO+ was mapped with angular resolutions ranging from 14 arcsec to 5 arcsec (or 0.01pc at the distance of HH2). The continuum shows an extended dust clump of mass 3.8Msun and temperature 22K, located downstream of the bright optical HH knots. However, a compact emission peak lies within ~0.01pc of the low-excitation H2-prominent shocks, with a luminosity consistent with local heating by the outflow. The HCO+ emission shows two velocity components: firstly, ambient-velocity gas lying in a region roughly corresponding to the dust clump, with abundance enhanced by a factor of a few close to the H2-prominent knots. Secondly a component of high-velocity emission (20 km/s linewidth), found mainly in a collimated jet linking the low-excitation HH objects. In this high-velocity jet, the line wings show an abundance ratio HCO+/CO proportional to v^2, with an HCO+ enhancement compared with ambient gas of up to 10^3 at the most extreme velocities. Such high abundances are consistent with models of shock chemistry in turbulent mixing layers at the interaction boundaries of jets. Extrapolating this effect to low velocities suggests that the more modest HCO+ enhancement in the clump gas could be caused by low velocity shocks. A UV precursor may not, therefore be necessary to explain the elevated HCO+ abundance in this gas.

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arXiv2002

A Hubble Space Telescope Lensing Survey of X-ray Luminous Galaxy Clusters: III. A Multiply-imaged Extremely Red Galaxy at z=1.6

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of EROJ003707+0909.5, the brightest of three gravitationally-lensed images of an Extremely Red Object (ERO) at z=1.6, in the field of the massive cluster A68 (z=0.255). We exploit the superlative resolution of our HST data and the enhanced spatial resolution and sensitivity afforded by the lens amplification to reconstruct the source-plane properties of this ERO. Our morphological and photometric analysis reveals that EROJ003707 is an L* early-type disk-galaxy and we estimate that ~10 per cent of EROs with (R-K)>=5.3 and K<=21 may have similar properties. The unique association of passive EROs with elliptical galaxies therefore appears to be too simplistic. We speculate on the evolution of EROJ003707: if gas continues to cool onto this galaxy in the manner predicted by hierarchical galaxy formation models, then by the present day, EROJ003707 could evolve into a very luminous spiral galaxy.

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arXiv2002

The Magnetic Field Structure in W51A

We present 850 micron imaging polarimetry of the W51A massive star forming region performed with SCUBA on the JCMT. From the polarimetry we infer the column-averaged magnetic field direction, projected onto the plane of the sky. We find that the magnetic field geometry in the region is complicated. We compare the field geometry with 6 cm and CS J=7-6 emission and determine that the magnetic field must be relatively weak and plays a passive role, allowing itself to be shaped by pressure forces and dynamics in the ionised and neutral gases. Comparisons are drawn between our data and 1.3 mm BIMA interferometric polarimetry data, from which we conclude that the magnetic field must increase in importance as we move to smaller scales and closer to sites of active star formation.

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arXiv2001

The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z=2.04

We present Ulysses and NEAR data from the detection of the short or intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT). The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN) and RXTE to an area of 50 arcmin^2. A fading optical counterpart was subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after the burst. The GRB lies at the border between the long-soft and the short-hard classes of GRBs. If GRB000301C belongs to the latter class, this would be the first detection of an afterglow to a short-hard burst. We present UBRI and JHK photometry from the time of the discovery until 11 days after the burst. Finally, we present spectroscopic observations of the optical afterglow obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The optical light curve is consistent with being achromatic from 2 to 11 days after the burst and exhibits a break. A broken power-law fit yields a shallow pre-break decay power-law slope of a_1=-0.72+-0.06, a break time of t_b=4.39+-0.26 days after the burst, and a post-break slope of a_2=-2.29+-0.17, which is best explained by a sideways expanding jet in an ambient medium of constant mean density. In the optical spectrum we find absorption features that are consistent with FeII, CIV, CII, SiII and Ly-a at a redshift of 2.0404+-0.0008. We find evidence for a curved shape of the spectral energy distribution of the observed afterglow. It is best fitted with a power-law spectral distribution with index b ~ -0.7 reddened by an SMC-like extinction law with A_V~0.1 mag. Based on the Ly-a absorption line we estimate the HI column density to be log(N(HI))=21.2+-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-a Absorbers.

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arXiv1999

Shocked Molecular Hydrogen from RNO91

We report the detection of the H2 v=1-0 S(1) line at 2.122um, from RNO91 in the L43 dark cloud, which is known to be a T Tauri star surrounded by a 1700AU disk structure (containing ices) and a weak outflow. The non-detection of the H2 v=2-1 S(1) line at 2.247um suggests shock excitation rather than fluorescence. The emission is extended spatially up to 9arcsec in the north-south direction. The line intensity peak (FWHM 3arcsec) corresponds to the star RNO91 which is embedded in a cocoon of gas and dust. The observed H2 emission from this cocoon may be attributed to embedded Herbig-Haro like knots. The H2 line flux in the central 2"X3"is estimated to be 7X10^{-14} ergs/sec/cm^{2}, which indicates a mass flow rate of 4X10^{-8} M_{sun}/yr. Furthermore, narrow band image taken through H2 1-0 S(1) filter is presented, which reveal a tilted disk and bipolar outflow structure that agrees with earlier observations and models. We show that this disk/outflow system is a unique case.

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