Monthly Notices of the Royal Astronomical Society

doi: 10.1111/j.1365-2966.2008.14118.xpmid: N/A

Dale, J. E.; Bonnell, I. A.

doi: 10.1111/j.1365-2966.2008.13802.xpmid: N/A

We present smoothed particle hydrodynamics simulations of protoclusters including the effects of the stellar winds from massive stars. Using a particle-injection method, we investigate the effect of structure in close proximity to the wind sources and the short-time-scale influence of winds on protoclusters. We find that the structures such as discs and gaseous filaments have a strong collimating effect on winds. By a different technique of injecting momentum from point sources into our simulations, we compare the large-scale and long-term effects of isotropic and intrinsically collimated winds on protoclusters and find them to be similar, although the collimated winds take longer to exert a significant influence. We find that both types of wind are able to dramatically slow the global star formation process, but that the time-scale on which they can expel significant quantities of mass from the cluster is rather long (approaching 10 free-fall times). Clusters may then experience rapid star formation very early in their lifetimes, before switching to a mode where gas is gradually expelled, while star formation proceeds much more slowly over many free-fall times. This complicates any conclusions regarding slow star formation derived from measuring the star formation efficiency per free-fall time. We find that estimates of the efficacy of winds in dispersing clusters derived simply from the total wind momentum flux may not be very reliable. This is due to material being expelled from deep within stellar potential wells, often to velocities well in excess of the cluster escape velocity, and also to the loss of momentum flux through holes in the gas distribution. Winds have little effect on the accretion-driven stellar initial mass function (IMF) except at the very high mass end, where they serve to prevent some of the most massive objects accreting more material. Feedback does not result in the formation of further massive stars through the monolithic collapse of massive cores. We also find that the morphology of the gas, the rapid motions of the wind sources and the action of large-scale accretion flows prevent the formation of bubble-like structures. These effects may make it difficult to discern the influence of winds on very young clusters.

De Lucia, Gabriella; Helmi, Amina

doi: 10.1111/j.1365-2966.2008.13862.xpmid: N/A

We use a series of high-resolution simulations of a ‘Milky Way’ halo coupled to semi-analytic methods to study the formation of our own Galaxy and its stellar halo. The physical properties of our model Milky Way, as well as the age and metallicity distribution of stars in the different components, are in relatively good agreement with observational measurements. Assuming that the stellar halo builds up from the cores of the satellite galaxies that merged with the Milky Way over its lifetime, we are able to study the physical and structural properties of this component. In agreement with previous work, we find that the largest contribution to the stellar halo should come from a few relatively massive (108– 1010M⊙) satellites accreted at early times. Our ‘stellar halo’ does not exhibit any clear metallicity gradient, but higher metallicity stars are more centrally concentrated than stars of lower abundance. This indicates that the probability of observing low-metallicity halo stars increases with distance from the Galactic Centre. We find that the proposed ‘dual’ nature of the Galactic stellar halo can be explained in our model as a result of a mass–metallicity relation imprinted in the building blocks of this component.

Dovčiak, M.; Muleri, F.; Goosmann, R. W.; Karas, V.; Matt, G.

doi: 10.1111/j.1365-2966.2008.13872.xpmid: N/A

Thermal emission from the accretion disc around a black hole can be polarized, due to Thomson scattering in a disc atmosphere. In Newtonian space, the polarization angle must be either parallel or perpendicular to the projection of the disc axis on the sky. As first pointed out by Stark and Connors in 1977, General Relativity effects strongly modify the polarization properties of the thermal radiation as observed at infinity. Among these effects, the rotation of the polarization angle with energy is particularly useful as a diagnostic tool.In this paper, we extend the Stark and Connors calculations by including the spectral hardening factor, several values of the optical depth of the scattering atmosphere and rendering the results to the expected performances of planned X-ray polarimeters. In particular, to assess the perspectives for the next generation of X-ray polarimeters, we consider the expected sensitivity of the detectors on board the planned POLARIX and International X-ray Observatory missions. We assume the two cases of a Schwarzschild and an extreme Kerr black hole with a standard thin disc and a scattering atmosphere. We compute the expected polarization degree and the angle as functions of the energy as they could be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. We assume the thermal emission dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105 in the thermal state, we calculate the observed polarization.

Frinchaboy, P. M.; Marino, A. F.; Villanova, S.; Carraro, G.; Majewski, S. R.; Geisler, D.

doi: 10.1111/j.1365-2966.2008.13875.xpmid: N/A

We present new high-resolution spectroscopy from which we derive abundances and radial velocities for stars in the field of the open cluster Tombaugh 2 (To2), which has been suggested to be one of a group of clusters previously identified with the Galactic Anticenter Stellar Structure (GASS) (also known as the Monoceros stream). Using Very Large Telescope/Fibre Large Array Multi-Element Spectrograph (VLT/FLAMES) with the Ultra-Violet and Visible Echelle Spectrograph (UVES) and GIRAFFE spectrographs, we find a radial velocity (RV) of 〈Vr〉= 121 ± 0.4 km s−1 using 18 To2 cluster stars; this is in agreement with previous studies, but at higher precision. We also make the first measurement of To2's velocity dispersion, which is σint= 1.8 ± 0.3 km s−1. Our abundance analysis of RV-selected members finds that To2 is more metal rich than previous studies have found; moreover, unlike the previous work, our larger sample also reveals that stars with the velocity of the cluster show a relatively large spread in chemical properties (e.g. Δ[Fe/H] > 0.2). This is the first time a possible abundance spread has been observed in an open cluster, though this is one of several possible explanations for our observations. While there is an apparent trend of [α/Fe] with [Fe/H], the distribution of abundances of these ‘RV cluster members’ also may hint at a possible division into two primary groups with different mean chemical characteristics – namely (〈[Fe/H]〉, 〈[Ti/Fe]〉) ∼ (−0.06, +0.02) and (−0.28, +0.36). Isochrone fitting to the colour–magnitude distribution of apparent To2 members yields an age of 2.0 Gyr, E(B−V) = 0.3, and (m−M)0= 14.5 or d= 7.9 kpc for both populations – parameters that are within the range of previous findings. Based on position and kinematics To2 is a likely member of the GASS/Monoceros stream, which makes To2 the second star cluster within the originally proposed GASS/Monoceros family after NGC 2808 to show some evidence for internal population dispersions. However, we explore other possible explanations for the observed spread in abundances and two possible subpopulations, with the most likely explanation being that the metal poor ([Fe/H]=−0.28), more centrally concentrated population being the true To2 clusters stars and the metal-rich ([Fe/H]=−0.06) population being an overlapping, and kinematically associated, but ‘cold’(σV < 2 km s−1) stellar stream at Rgc≥ 15 kpc.

Ramos-Larios, G.; Phillips, J. P.; Cuesta, L.

doi: 10.1111/j.1365-2966.2008.13886.xpmid: N/A

The bipolar planetary nebula NGC 650-1 has been imaged in Hα, Hβ, [N ii]λ6583 Å and [S ii]λλ6717+6731 Å. These results are used to map the variation of extinction over the interior regions of the shell, where we find evidence for a gradual increase in AV between the SW and NE limits of the central emission bar. It is argued that this is likely to arise from dust associated with the nebular envelope itself. We have also produced a map of electron densities over this region, and find evidence for marked variations in ne over a scale of ∼5 arcsec, a variation which is probably related to the clumpy emission structure observed in low-excitation images. We finally present contour mapping of mid-infrared Spitzer images of the source at 3.6, 4.5, 5.8 and 8 μm. These show that the central bar has a more or less uniform emission structure, although with evidence for condensations which are similar to, and in most cases coincidental with, comparable low-excitation structures noted in [N ii]. There is also evidence for an increase in 8/4.5 μm and 5.8/4.5 μm emission ratios with distance from the nucleus, and for an extension of 8 and 5.8 μm emission beyond the ionized regime; a trend which is attributed to strong polycyclic aromatic hydrocarbon band emission within the nebular photodissociative regime.

Iliev, Ilian T.; Shapiro, Paul R.; McDonald, Patrick; Mellema, Garrelt; Pen, Ue-Li

doi: 10.1111/j.1365-2966.2008.13879.xpmid: N/A

Observations of high-redshift Lyα sources are a major tool for studying the high-redshift universe and are one of the most promising ways to constrain the later stages of reionization. The understanding and interpretation of the data is far from straightforward, however. We discuss the effect of the reionizing intergalactic medium (IGM) on the observability of Lyα sources based on large simulations of early structure formation with radiative transfer. This takes into account self-consistently the reionization history, density, velocity and ionization structures and non-linear source clustering. We find that all fields are highly anisotropic and as a consequence there are very large variations in opacity among the different lines of sight. The velocity effects, from both infall and source peculiar velocity are most important for the luminous sources, affecting the line profile and depressing the bright end of the luminosity function. The line profiles are generally asymmetric and the line centres of the luminous sources are always absorbed due to the high density of the local IGM. For both luminous and average sources the damping wing effects are of similar magnitude and remain significant until fairly late, when the IGM is ionized between 30 and 70 per cent by mass.The ionizing flux in the ionized patch surrounding a high-density peak is generally strongly dominated, particularly at late times, by the cluster of faint sources, rather than the central massive galaxy. Our results reproduce well the observed mean opacity of the IGM at z∼ 6. The IGM absorption does not change appreciably the correlation function of sources at high redshift. Our derived luminosity function assuming constant mass-to-light ratio provides an excellent match to the shape of the observed luminosity function at z= 6.6 with faint-end slope of α=−1.5. The resulting mass-to-light ratio implies that the majority of sources responsible for reionization are too faint to be observed by the current surveys.

Kun, M.; Balog, Z.; Mizuno, N.; Kawamura, A.; Gáspár, A.; Kenyon, S. J.; Fukui, Y.

doi: 10.1111/j.1365-2966.2008.13898.xpmid: N/A

We present results of optical spectroscopic and photometric observation of the pre-main-sequence stars associated with the cometary shaped dark cloud Lynds 1622, and 12CO and 13CO observations of the cloud. We determined the effective temperatures and luminosities of 14 pre-main-sequence stars associated with the cloud from their positions in the Hertzsprung–Russell diagram, as well as constructed their spectral energy distributions using optical, 2MASS and Spitzer IRAC and MIPS data. We derived physical parameters of L 1622 from the molecular observations. Our results are not compatible with the assumption that L 1622 lies on the near side of the Orion–Eridanus loop, but suggest that L 1622 is as distant as Orion B. At a distance of 400 pc the mass of the cloud, derived from our 12CO data, is 1100 M⊙, its star formation efficiency is ∼1.8 per cent, and the average age of its low-mass pre-main-sequence star population is about 1 Myr.

Reddy, Bacham E.; Lambert, David L.

doi: 10.1111/j.1365-2966.2008.13905.xpmid: N/A

An abundance analysis is presented of 60 metal-poor stars drawn from catalogues of nearby stars provided by Arifyanto et al. and Schuster et al. In an attempt to isolate a sample of metal-weak thick disc stars, we applied the kinematic criteria Vrot≥ 100 km s−1, |ULSR| ≤ 140 km s−1 (LSR – local standard of rest) and |WLSR| ≤ 100 km s−1. 14 stars satisfying these criteria and having [Fe/H]≤−1.0 are included in the sample of 60 stars. Eight of the 14 have [Fe/H]≥−1.3 and may be simply thick disc stars of slightly lower than average [Fe/H]. The other six have [Fe/H] from −1.3 to −2.3 and are either metal-weak thick disc stars or halo stars with kinematics mimicking those of the thick disc. The sample of 60 stars is completed by eight thick disc stars, 20 stars of a hybrid nature (halo or thick disc stars) and 18 stars with kinematics distinctive of the halo.

Davé, Romeel; Oppenheimer, Benjamin D.; Sivanandam, Suresh

doi: 10.1111/j.1365-2966.2008.13906.xpmid: N/A

We examine metal and entropy content in galaxy groups having TX≈ 0.5–2 keV in cosmological hydrodynamic simulations. Our simulations include a well-constrained prescription for galactic outflows following momentum-driven wind scalings, and a sophisticated chemical evolution model. Our simulation with no outflows reproduces observed iron abundances in X-ray emitting gas, but the oxygen abundance is too low; including outflows yields iron and oxygen abundances in good agreement with data. X-ray measures of [O/Fe] primarily reflect metal distribution mechanisms into hot gas, not the ratio of Type Ia to Type II supernovae within the group. Iron abundance increases by ∼×2 from z∼ 1 to 0 independent of group size, consistent with that seen in clusters, while [O/Fe] drops by ∼30 per cent. Core entropy versus temperature is elevated over self-similar predictions regardless of outflows due to radiative cooling removing low-entropy gas, but outflows provide an additional entropy contribution below 1 keV. This results in a noticeable break in the LX–TX relation below ∼1 keV, as observed. Entropy at R500 is also in good agreement with data, and is unaffected by outflows. Importantly, outflows serve to reduce the stellar content of groups to observed levels. Specific energy injection from outflows drops with group mass, and exceeds the thermal energy for ≲0.5-keV systems. Radial profiles from simulations are in broad agreement with observations, but there remain non-trivial discrepancies that may reflect an excess of late-time star formation in central group galaxies in our simulations. Our model with outflows suggests a connection between physical processes of galaxy formation and both pre-heating and enrichment in intragroup gas, though more definitive conclusions must await a model that simultaneously suppresses cooling flows as observed.