Measurement of charge exchange and X-ray emission cross sections for solar wind-comet interactions

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

Observations towards early-type stars in the ESO-POP Survey - II. Searches for intermediate- and high-velocity clouds

We present Ca it K and Ti it optical spectra of early-type stars taken mainly from the ultraviolet and visual echelle spectrograph (LIVES) Paranal Observatory Project, plus H 1 21-cm spectra, from the Vila-Elisa and Leiden-Dwingeloo Surveys, which are employed to obtain distances to intermediate- and high-velocity clouds (IHVCs). H I emission at a velocity of -117 km s(-1) towards the sightline HD 30677 (l, b = 190 degrees.2, -22 degrees.2) with column density -1.7 x 10(19) cm(-2) has no corresponding Ca Pi K absorption in the LIVES spectrum, which has a signal-to-noise ratio (S/N) of 610 per resolution element. The star has a spectroscopically determined distance of 2.7 kpc, and hence sets this as a firm lower distance limit towards Anti-Centre cloud ACII. Towards another sightline (HD 46185 with 1, b = 222 0, -10 degrees.1), H1 at a velocity of +122 km s(-1) and column density of 1.2 x 10(19) cm(-2) is seen. The corresponding Ca Pi K spectrum has a S/N of 780, although no absorption is observed at the cloud velocity. This similarly places a firm lower distance limit of 2.9 kpc towards this parcel of gas that may be an intermediate-velocity (IV) cloud. The lack of IV Ca it absorption towards HD 196426 (1, b = 45 degrees.8, -23 degrees.3) at a S/N of 500 reinforces a lower distance limit of -700 pc towards this part of complex gp, where the H I column density is 1.1 x 1019 cm(-2) and velocity is +78 km s(-1). Additionally, no IV Cart is seen in absorption in the spectrum of HD 19445, which is strong in H I with a column density of 8 x 10(19) cm(-2) at a velocity of - -42 km s(-1), placing a firm although uninteresting lower distance limit of 39 pc to this part of IV South. Finally, no high-velocity Call K absorption is seen towards HD 115363 (l, b = 306.0,-1.0) at a S/N of 410, placing a lower distance of -3.2 kpc towards the HVC gas at velocity of - +224 km s(-1) and WE column density of 5.2 x 10(19) cm(-2). This gas is in the same region of the sky as complex WE (Wakker 2001), but at higher velocities. The non-detection of Ca it K absorption sets a lower distance of -3.2 kpc towards the HVC, which is unsurprising if this feature is indeed related to the Magellanic System.

The evolution of solar systems

The recent detection of extra-solar planets and the increasing ability of modern instruments to image discs around young stars has brought a renewed interest in the formation of solar systems. In this article, I shall briefly review what we know about extra-solar planets and the physical structure of protostellar discs. One of the most powerful means of studying these disc systems is to observe the rotational line emission from molecules which can give information on physics and dynamics. At present, the observations are relatively crude but future instruments should be able to resolve molecular structures in the disc around nearby stars. As a prelude to these observations, I discuss some conceptually simple, although numerically challenging, models of the physical and chemical processes involved in determining the molecular distributions.

Rotation periods of exoplanet host stars

The stellar rotation periods of 10 exoplanet host stars have been determined using newly analysed CaII H&K flux records from the Mount Wilson Observatory and Strömgren b, y photometric measurements from Tennessee State University's automatic photometric telescopes at the Fairborn Observatory. Five of the rotation periods have not previously been reported, with that of HD 130322 very strongly detected at Prot = 26.1 +/- 3.5 d. The rotation periods of five other stars have been updated using new data. We use the rotation periods to derive the line-of-sight inclinations of the stellar rotation axes, which may be used to probe theories of planet formation and evolution when combined with the planetary orbital inclination found from other methods. Finally, we estimate the masses of 14 exoplanets under the assumption that the stellar rotation axis is aligned with the orbital axis. We calculate the mass of HD 92788 b (28 MJ) to be within the low-mass brown dwarf regime and suggest that this object warrants further investigation to confirm its true nature.

Massive stars exploding in a He-rich circumstellar medium - III. SN 2006jc: infrared echoes from new and old dust in the progenitor CSM

We present near- (NIR) and mid-infrared (MIR) photometric data of the Type Ibn supernova (SN) 2006jc obtained with the United Kingdom Infrared Telescope (UKIRT), the Gemini North Telescope and the Spitzer Space Telescope between days 86 and 493 post-explosion. We find that the IR behaviour of SN 2006jc can be explained as a combination of IR echoes from two manifestations of circumstellar material. The bulk of the NIR emission arises from an IR echo from newly condensed dust in a cool dense shell (CDs) produced by the interaction of the ejecta Outward shock with a dense shell of circumstellar material ejected by the progenitor in a luminous blue variable (LBV)-like outburst about two years prior to the SN explosion. The CDs dust mass reaches a modest 3.0 x 10(-4) M-circle dot by day 230. While dust condensation within a CDs formed behind the ejecta inward shock has been proposed before for one event (SN 1998S), SN 2006jc is the first one showing evidence for dust condensation in a CDs formed behind the ejecta outward shock in the circumstellar material. At later epochs, a substantial and growing contribution to the IR fluxes arises from an IR echo from pre-existing dust in the progenitor wind. The mass of the pre-existing circumstellar medium (CSM) dust is at least similar to 8 x 10(-3) M-circle dot. This paper therefore adds to the evidence that mass-loss from the progenitors of core-collapse SNe could be a major source of dust in the Universe. However, yet again, we see no direct evidence that the explosion of an SN produces anything other than a very modest amount of dust.

Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and tau Bootis

The stars 51 Pegasi and tau Bootis show radial velocity variations that have been interpreted as resulting from companions with roughly Jovian mass and orbital periods of a few days. Gray and Gray & Hatzes reported that the radial velocity signal of 51 Peg is synchronous with variations in the shape of the line lambda 6253 Fe I; thus, they argue that the velocity signal arises not from a companion of planetary mass but from dynamic processes in the atmosphere of the star, possibly nonradial pulsations. Here we seek confirming evidence for line shape or strength variations in both 51 Peg and tau Boo, using R = 50,000 observations taken with the Advanced Fiber Optic Echelle. Because of our relatively low spectral resolution, we compare our observations with Gray's line bisector data by fitting observed line profiles to an expansion in terms of orthogonal (Hermite) functions. To obtain an accurate comparison, we model the emergent line profiles from rotating and pulsating stars, taking the instrumental point-spread function into account. We describe this modeling process in detail. We find no evidence for line profile or strength variations at the radial velocity period in either 51 Peg or in tau Boo. For 51 Peg, our upper limit for line shape variations with 4.23 day periodicity is small enough to exclude with 10 sigma confidence the bisector curvature signal reported by Gray & Hatzes; the bisector span and relative line depth signals reported by Gray are also not seen, but in this case with marginal (2 sigma ) confidence. We cannot, however, exclude pulsations as the source of 51 Peg's radial velocity variation because our models imply that line shape variations associated with pulsations should be much smaller than those computed by Gray & Hatzes; these smaller signals are below the detection limits both for Gray & Hatzes's data and for our own. tau Boo's large radial velocity amplitude and v sin i make it easier to test for pulsations in this star. Again we find no evidence for periodic line shape changes, at a level that rules out pulsations as the source of the radial velocity variability. We conclude that the planet hypothesis remains the most likely explanation for the existing data.

Molecular abundances in the magellanic clouds .1. A multiline study of five cloud cores

Nine H II regions of the LMC were mapped in (CO)-C-13(1-0) and three in (CO)-C-12(1-0) to study the physical properties of the interstellar medium in the Magellanic Clouds. For N113 the molecular core is found to have a peak position which differs from that of the associated H II region by 20 ''. Toward this molecular core the (CO)-C-12 and (CO)-C-13 peak T-MB line temperatures of 7.3 K and 1.2 K are the highest so far found in the Magellanic Clouds. The molecular concentrations associated with N113, N44BC, N159HW, and N214DE in the LMC and LIRS 36 in the SMC were investigated in a variety of molecular species to study the chemical properties of the interstellar medium. I(HCO+)/I(HCN) and I(HCN)/I(HNC) intensity ratios as well as lower limits to the I((CO)-C-13)/I((CO)-O-18) ratio were derived for the rotational 1-0 transitions. Generally, HCO+ is stronger than HCN, and HCN is stronger than HNC. The high relative HCO+ intensities are consistent with a high ionization flux from supernovae remnants and young stars, possibly coupled with a large extent of the HCO+ emission region. The bulk of the HCN arises from relatively compact dense cloud cores. Warm or shocked gas enhances HCN relative to HNC. From chemical model calculations it is predicted that I(HCN)/I(HNC) close to one should be obtained with higher angular resolution (less than or similar to 30 '') toward the cloud cores. Comparing virial masses with those obtained from the integrated CO intensity provides an H-2 mass-to-CO luminosity conversion factor of 1.8 x 10(20) mol cm(-2) (K km s(-1))(-1) for N113 and 2.4 x 10(20) mol cm(-2) (K km s(-1))(-1) for N44BC. This is consistent with values derived for the Galactic disk.

Early-type stars observed in the ESO UVES Paranal Observatory Project - III. Sub-parsec and AU-scale structure in the interstellar medium

UVES interstellar observations from the Paranal Observatory Project are presented for early-type stars located in the line of sight to the nearby open clusters IC 2391 (Omni Vel) and NGC 6475 (M7), with spectroscopic resolution R similar to 80 000 and signal-to-noise ratios in the Ti II (3383 angstrom), Ca II K, CH+ (4232 angstrom), Na I D and K I (7698 angstrom) lines of several hundred. The sightlines are a mixture of cluster and non-cluster objects. A total of 22 early-type stars (A and B type) are present in our sample towards IC 2391, with 21 towards NGC 6475/M7, and enable us to probe for differences in column density on scales from similar to 0.07 to 7.3 and similar to 0.05 to 4.9 pc in the respective clusters. Additionally, towards Praesepe the Na I D interstellar variation only is probed towards 13 sightlines and transverse scales of similar to 0.16-10.7 pc at R = 70 000. Towards IC 2391 variations are found in Ti II, Ca II K and Na I D column density in different sightlines of up to 0.7, 1.0 and 1.8 dex (excluding one star), respectively. This kind of variability correlates well with the Hipparcos parallax of the objects, and probes structure within the Local Bubble. For cluster-only objects the variations are 0.3, 0.3 and 0.5 dex, respectively. For the field of view towards NGC6475 the corresponding maximum variations are somewhat smaller, being 0.5, 0.3, 0.8 and 1.0 dex for Ti II, Ca II K, Na I and K I, respectively, for all objects and 0.4, 0.2, 0.6 and 0.7 dex for the cluster-only objects. These are uncorrelated with parallax, and again demonstrate that Ca II K tends to be more smoothly distributed than Na I D. A few likely cluster sightlines show evidence for CH+ and variations in this molecular species of a factor of 10 in equivalent width over sub-pc scales. Towards Praesepe variation in interstellar Na I D is small, being a maximum of only similar to 0.4 dex (including measurement errors), but with fewer sightlines studied. Overall, the scatter in the data is similar for the singly ionized species Ti II and Ca II, lending more support to the hypothesis that these two species sample similar parts of the interstellar medium (ISM). This also appears to be the case for the neutral species Na I D and K I in the one cluster studied. Finally, multiple-epoch observations from a variety of archive sources are used to search for astronomical unit (au) scale structure in the ISM towards 46 sightlines. There are tentative indications of structure on scales of tens to thousands of au for three sightlines. Future observations will confirm the veracity or otherwise of the time-variable components and others presented.

An analysis of VUV C (IV) emission from the JET divertor giving measurements of electron temperatures

A description of the radiation emitted by impurities from within a plasma is crucial if spectral line intensities are to be used in detailed studies, such as the analysis of impurity transport. The simplest and most direct check that can be made on measurements of line intensities is to analyse their ratios with other lines from the same ion. This avoids uncertainties in determining the volume of the emitting plasma and the absolute sensitivity calibration of the spectrometer and, in some cases, the need even for accurate measurements of parameters such as electron density. Consistency is required between the measured line intensity ratios and the theoretical values. The expected consistency has not been found for radiation emitted from the JET scrape-off layer (e.g. Lawson et al 2009a JINST 4 P04013), meaning that the description of the spectral line intensities of impurity emission from the plasma edge is incomplete. In order to gain further understanding of the discrepancies, an analysis has been carried out for emission from the JET divertor plasma and this is reported in this paper. Carbon was the main low Z intrinsic impurity in JET and an analysis of spectral line intensity ratios has been made for the C (IV) radiation emitted from the JET divertor. In this case, agreement is found between the measured and theoretical ratios to a very high accuracy, namely to within the experimental uncertainty of similar to +/- 10%. This confirms that the description of the line intensities for the present observations is complete. For some elements and ionization stages, an analysis of line intensity ratios can lead to the determination of parameters such as the electron temperature of the emitting plasma region and estimates of the contribution of recombination to the electron energy level populations. This applies to C (IV) and, to show the value and possibilities of the spectral measurements, these parameters have been calculated for a database of Ohmic and additionally heated phases of a large number of pulses. The importance of dielectronic, radiative and charge-exchange recombination as well as ionization has been investigated. In addition, the development of T-e throughout two example discharges is illustrated. The presented results indicate a number of areas for further investigation.

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.

The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

We present a comprehensive study of the observational dependence of the mass-loss rate in stationary stellar winds of hot massive stars on the metal content of their atmospheres. The metal content of stars in the Magellanic Clouds is discussed, and a critical assessment is given of state-of-the-art mass-loss determinations of OB stars in these two satellite systems and the Milky-Way. Assuming a power-law dependence of mass loss on metal content,. M. Z(m), and adopting a theoretical relation between the terminal flow velocity and metal content, v(infinity). Z(0.13) (Leitherer et al. 1992, ApJ, 401, 596), we find m = 0.83 +/- 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 105.2 L circle dot. Within the errors, this result is in agreement with the prediction m = 0.69 +/- 0.10 by Vink et al. (2001, A& A, 369, 574). Absolute empirical values for the mass loss, based on Ha and ultraviolet (UV) wind lines, are found to be a factor of two higher than predictions in this high luminosity regime. If this difference is attributed to inhomogeneities in the wind, and this clumping does not impact the predictions, this would imply that luminous O and early-B stars have clumping factors in their Ha and UV line forming regions of about a factor of four. For lower luminosity stars, the winds are so weak that their strengths can generally no longer be derived from optical spectral lines (essentially Ha) and one must currently rely on the analysis of UV lines. We confirm that in this low-luminosity domain the observed Galactic WLR is found to be much steeper than expected from theory (although the specific sample is rather small), leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L similar to 10(4.75) L circle dot, the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars would turn out to be overestimated as a result of wind clumping, but the degree of clumping would be rather independent of metallicity, the scalings derived in this study are expected to remain correct.

The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud

We have studied the optical spectra of a sample of 28 O- and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star forming region N11. Our observations sample the central associations of LH9 and LH10, and the surrounding regions. Stellar parameters are determined using an automated fitting method ( Mokiem et al. 2005), which combines the stellar atmosphere code fastwind ( Puls et al. 2005) with the genetic algorithm based optimisation routine PIKAIA ( Charbonneau 1995). We derive an age of 7.0 +/- 1.0 and 3.0 +/- 1.0 Myr for LH9 and LH10, respectively. The age difference and relative distance of the associations are consistent with a sequential star formation scenario in which stellar activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2. From helium and hydrogen line fitting we find the hottest three of these stars to be similar to 49- 54 kK ( compared to similar to 45- 46 kK for O3 stars). Detailed determination of the helium mass fraction reveals that the masses of helium enriched dwarfs and giants derived in our spectroscopic analysis are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally enhanced mixing leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. ( 2006). For bright giants and supergiants no such mass discrepancy is found; these stars therefore appear to follow tracks of modestly or non-rotating objects. The set of programme stars was sufficiently large to establish the mass loss rates of OB stars in this Z similar to 1/2 Z(circle dot) environment sufficiently accurate to allow for a quantitative comparison with similar objects in the Galaxy and the SMC. The mass loss properties are found to be intermediate to massive stars in the Galaxy and SMC. Comparing the derived modified wind momenta D-mom as a function of luminosity with predictions for LMC metallicities by Vink et al. ( 2001) yields good agreement in the entire luminosity range that was investigated, i.e. 5.0

The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region

We present new observations of 470 stars using the Fibre Large Array Multi-Element Spectrograph ( FLAMES) instrument in fields centered on the clusters NGC330 and NGC346 in the Small Magellanic Cloud (SMC), and NGC2004 and the N11 region in the Large Magellanic Cloud (LMC). A further 14 stars were observed in the N11 and NGC330 fields using the Ultraviolet and Visual Echelle Spectrograph (UVES) for a separate programme. Spectral classifications and stellar radial velocities are given for each target, with careful attention to checks for binarity. In particular, we have investigated previously unexplored regions around the central LH9/LH10 complex of N11, finding similar to 25 new O-type stars from our spectroscopy. We have observed a relatively large number of Be-type stars that display permitted Fe II emission lines. These are primarily not in the cluster cores and appear to be associated with classical Be-type stars, rather than pre main-sequence objects. The presence of the Fe II emission, as compared to the equivalent width of Ha, is not obviously dependent on metallicity. We have also explored the relative fraction of Be- to normal B-type stars in the field-regions near to NGC330 and NGC2004, finding no strong evidence of a trend with metallicity when compared to Galactic results. A consequence of service observations is that we have reasonable time-sampling in three of our FLAMES fields. We find lower limits to the binary fraction of O- and early B-type stars of 23 to 36%. One of our targets (NGC346-013) is especially interesting with a massive, apparently hotter, less luminous secondary component.

Blue luminous stars in nearby galaxies: Quantitative spectral analysis of M33 B-type supergiant stars

We present the detailed spectral analysis of a sample of M33 B-type supergiant stars, aimed at the determination of their fundamental parameters and chemical composition. The analysis is based on a grid of non-LTE metal line-blanketed model atmospheres including the effects of stellar winds and spherical extension computed with the code FASTWIND. Surface abundance ratios of C, N, and O are used to discuss the chemical evolutionary status of each individual star. The comparison of observed stellar properties with theoretical predictions of massive star evolutionary models shows good agreement within the uncertainties of the analysis. The spatial distribution of the sample allows us to investigate the existence of radial abundance gradients in the disk of M33. The comparison of stellar and H II region O abundances ( based on direct determinations of the electron temperature of the nebulae) shows good agreement. Using a simple linear radial representation, the stellar oxygen abundances result in a gradient of -0.0145 +/- 0.005 dex arcmin(-1) (or -0.06 +/- 0.02 dex kpc(-1)) up to a distance equal to similar to 1.1 times the isophotal radius of the galaxy. A more complex representation cannot be completely discarded by our stellar sample. The stellar Mg and Si abundances follow the trend displayed by O abundances, although with shallower gradients. These differences in gradient slope cannot be explained at this point. The derived abundances of the three alpha-elements yield solar metallicity in the central regions of the disk of M33. A comparison with recent planetary nebula data from Magrini and coworkers indicates that the disk of M33 has not suffered from a significant O enrichment in the last 3 Gyr.