A rotating helical filament in the L1251 dark cloud

(Abridged) Aims. We derive the physical properties of a filament discovered in the dark cometary-shaped cloud L1251. Methods. Mapping observations in the NH3(1,1) and (2,2) inversion lines, encompassing 300 positions toward L1251, were performed with the Effelsberg 100-m telescope at a spatial resolution of 40 arcsec and a spectral resolution of 0.045 km/s. Results. The filament L1251A consists of three condensations (alpha, beta, and gamma) of elongated morphology, which are combined in a long and narrow structure covering a 38 arcmin by 3 arcmin angular range. The opposite chirality (dextral and sinistral) of the alpha+beta and gamma condensations indicates magnetic field helicities of two types, negative and positive, which were most probably caused by dynamo mechanisms. We estimated the magnetic Reynolds number Rm > 600 and the Rossby number R < 1, which means that dynamo action is important.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in A&

The baryon density at z=0.9-1.9 - Tracing the warm-hot intergalactic medium with broad Lyman alpha absorption

We present an analysis of the Lyman alpha forests of five quasar spectra in the near UV. Properties of the intergalactic medium (IGM) at an intermediate redshift interval (0.9 < z < 1.9) are studied. The amount of baryons in the diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are traced to get constraints on the redshift evolution of the different phases of the intergalactic gas. The baryon density of the diffuse IGM is determined with photoionisation calculations under the assumption of local hydrostatic equilibrium. We assume that the gas is ionised by a metagalactic background radiation with a Haardt & Madau (2001) spectrum. The WHIM is traced with broad Lyman alpha (BLA) absorption. The properties of a number of BLA detections are studied. Under the assumption of collisional ionisation equilibrium a lower limit to the baryon density could be estimated. It is found that the diffuse photoionised IGM contains at least 25% of the total baryonic matter at redshifts 1 < z < 2. For the WHIM a lower limit of 2.4% could be determined. Furthermore the data indicates that the intergalactic gas is in a state of evolution at z=1.5. We confirm that a considerable part of the WHIM is created between z=1 and z=2.Comment: 6 pages, 1 figure, accepted for publication in A&

Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission

(Abridged) Aims. We intend to derive statistical properties of stochastic gas motion inside the dense low mass star forming molecular cores traced by NH3(1,1) and (2,2) emission lines. Methods. We use the spatial two-point autocorrelation (ACF) and structure functions calculated from maps of the radial velocity fields. Results. We find oscillating ACFs which eventually decay to zero with increasing lags on scales of 0.04 <= l <= 0.5 pc. The current paradigm supposes that the star formation process is controlled by the interplay between gravitation and turbulence, the latter preventing molecular cores from a rapid collapse due to their own gravity. Thus, oscillating ACFs may indicate a damping of the developed turbulent flows surrounding the dense but less turbulent core - a transition to dominating gravitational forces and, hence, to gravitational collapse.Comment: 11 pages, 16 figures, 3 tables, to be published in Astronomy and Astrophysic

A New Version of Reimers' law of Mass Loss Based on a Physical Approach

We present a new semi-empirical relation for the mass loss of cool stellar winds, which so far has frequently been described by "Reimers' law". Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spill-over of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on effective temperature. The new relation is tested and sensitively calibrated by modelling the blue end of the Horizontal Branch of globular clusters. The most significant difference from mass loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter

Cosmological Reionization

In popular cosmological scenarios, some time beyond a redshift of 10, stars within protogalaxies created the first heavy elements; these systems, together perhaps with an early population of quasars, generated the ultraviolet radiation and mechanical energy that reheated and reionized the cosmos. The history of the Universe during and soon after these crucial formative stages is recorded in the all-pervading intergalactic medium (IGM), which contains most of the ordinary baryonic material left over from the big bang. Throughout the epoch of structure formation, the IGM becomes clumpy and acquires peculiar motions under the influence of gravity, and acts as a source for the gas that gets accreted, cools, and forms stars within galaxies, and as a sink for the metal enriched material, energy, and radiation which they eject.Comment: LateX, 13 pages, 4 figures, slightly revised version (corrected several typos), to appear in Phil. Trans. R. Soc. London A (2000) 35

Finding the First Stars: The Hamburg/ESO Objective Prism Survey

We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by automatic spectral classification. Extensive simulations show that the selection efficiency for turnoff stars of [Fe/H]25% at B<16.5. Since the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992), the HES offers the possibility to efficiently increase the total number of metal-poor stars by at least a factor of 4.Comment: To appear in: Proceedings of ESO/MPA conference "The First Stars". 2 pages, 1 figur

Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9

The shape of the UV ionizing background is reconstructed from optically thin metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143, and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are analyzed by means of the Monte Carlo Inversion method completed with the spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 < z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z < 1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua and the variability of the spectral shape comes from diversity of spectral indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered spectral shapes show intensity depression between 3 and 4 Ryd due to HeII Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium (true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the depth of this depression corresponds within 1-2sigma to the values directly measured from the HI/HeII Ly-alpha forest towards the quasars studied. The observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and eta can be explained by the combined action of variable spectral softness and differences in the mean gas density between the absorbing clouds. Neither of the recovered spectral shapes show features which can be attributed to the putative input of radiation from soft sources like starburst galaxies.Comment: 20 pages, 20 figures. Accepted for publication in A&