- ID:
- ivo://CDS.VizieR/J/A+A/627/A34
- Title:
- GC hot UV-bright stars model spectra
- Short Name:
- J/A+A/627/A34
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. We obtained FORS2 spectroscopy of 11 of these UV-selected objects (covering a range of -2.3<[Fe/H]<-1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. We find that metal-rich model spectra are required to analyse stars hotter than 40000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. Adding published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/MNRAS/445/1745
- Title:
- Giant Gemini GMOS survey. I.
- Short Name:
- J/MNRAS/445/1745
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present spectra of 163 quasars at z_em_>4.4 taken with the Gemini Multi Object Spectrometers, the largest publicly available sample of high-quality (signal-to-noise ratio S/N~20 per 1.85{AA} pixel) low-resolution (full width at half maximum FWHM~320km/s) rest-frame ultraviolet quasar spectra at these redshifts. The sample was selected from SDSS Data Release 7 with the aim to obtain high-S/N follow-up spectra of the intergalactic HI Lyman series and Lyman continuum absorption at high redshift. Each quasar was observed with the GMOS B600 (FWHM~320km/s) and R400 (FWHM~360km/s) gratings to cover the rest-frame wavelength range from ~850 to ~1500{AA}. Here we make the spectra available in reduced form. The accompanying paper presents a detailed description the sample and a measurement of the mean free path to Lyman limit photons in the IGM, based on stacked rest-frame quasar spectra.
- ID:
- ivo://CDS.VizieR/J/ApJ/797/50
- Title:
- Global energetics of solar flares. I.
- Short Name:
- J/ApJ/797/50
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first part of a project on the global energetics of solar flares and coronal mass ejections that includes about 400 M- and X-class flares observed with Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). We calculate the potential (E_p_), the nonpotential (E_np_) or free energies (E_free_=E_np_-E_p_), and the flare-dissipated magnetic energies (E_diss_). We calculate these magnetic parameters using two different NLFFF codes: the COR-NLFFF code uses the line-of-sight magnetic field component B_z_ from HMI to define the potential field, and the two-dimensional (2D) coordinates of automatically detected coronal loops in six coronal wavelengths from AIA to measure the helical twist of coronal loops caused by vertical currents, while the PHOT-NLFFF code extrapolates the photospheric three-dimensional (3D) vector fields. We find agreement between the two codes in the measurement of free energies and dissipated energies within a factor of <~3. The size distributions of magnetic parameters exhibit powerlaw slopes that are approximately consistent with the fractal-diffusive self-organized criticality model. The magnetic parameters exhibit scaling laws for the nonpotential energy, E_np_{propto}E_p_^1.02^, for the free energy, E_free_{propto}E_p_^1.7^ and E_free_{propto}B_{phi}_^1.0^L^1.5^, for the dissipated energy, E_diss_{propto}E_p_^1.6^ and E_diss_{propto}E_free_^0.9^ , and the energy dissipation volume, V{propto}E_diss_^1.2^. The potential energies vary in the range of E_p_=1x10^31^-4x10^33^erg, while the free energy has a ratio of E_free_/E_p_{approx}1%-25%. The Poynting flux amounts to F_flare_{approx}5x10^8^-10^10^erg/cm2/s during flares, which averages to F_AR_{approx}6x10^6^erg/cm2/s during the entire observation period and is comparable with the coronal heating rate requirement in active regions.
- ID:
- ivo://CDS.VizieR/J/ApJ/802/53
- Title:
- Global energetics of solar flares. II.
- Short Name:
- J/ApJ/802/53
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the second part of a project on the global energetics of solar flares and coronal mass ejections that includes about 400 M- and X-class flares observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during the first 3.5yr of its mission. In this Paper II we compute the differential emission measure (DEM) distribution functions and associated multithermal energies, using a spatially-synthesized Gaussian DEM forward-fitting method. The multithermal DEM function yields a significantly higher (by an average factor of ~14), but more comprehensive (multi-) thermal energy than an isothermal energy estimate from the same AIA data. We find a statistical energy ratio of E_th_/E_diss_~2-40% between the multithermal energy E_th_ and the magnetically dissipated energy E_diss_, which is an order of magnitude higher than the estimates of Emslie et al. (2012ApJ...759...71E). For the analyzed set of M- and X-class flares we find the following physical parameter ranges: L=10^8.2^-10^9.7^cm for the length scale of the flare areas, T_p_=10^5.7^-10^7.4^K for the DEM peak temperature, T_w_=10^6.8^-10^7.6^K for the emission measure-weighted temperature, n_p_=10^10.3^-10^11.8^/cm3 for the average electron density, EM_p_=10^47.3^-10^50.3^/cm3 for the DEM peak emission measure, and E_th_=10^26.8^-10^32.0^erg for the multithermal energies. The deduced multithermal energies are consistent with the RTV scaling law E_th,RTV_=73x10^-10^T_p_^3^L_p_^2^, which predicts extremal values of E_th,max_~1.5x10^33^erg for the largest flare and E_th,min_~1x10^24^erg for the smallest coronal nanoflare. The size distributions of the spatial parameters exhibit powerlaw tails that are consistent with the predictions of the fractal-diffusive self-organized criticality model combined with the RTV scaling law.
- ID:
- ivo://CDS.VizieR/J/ApJ/831/105
- Title:
- Global energetics of solar flares. IV. CME
- Short Name:
- J/ApJ/831/105
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This study entails the fourth part of a global flare energetics project, in which the mass m_cme_, kinetic energy E_kin_, and the gravitational potential energy E_grav_ of coronal mass ejections (CMEs) is measured in 399 M and X-class flare events observed during the first 3.5 years of the Solar Dynamics Observatory (SDO) mission, using a new method based on the extreme ultraviolet (EUV) dimming effect. EUV dimming is modeled in terms of a radial adiabatic expansion process, which is fitted to the observed evolution of the total emission measure of the CME source region. The model derives the evolution of the mean electron density, the emission measure, the bulk plasma expansion velocity, the mass, and the energy in the CME source region. The EUV dimming method is truly complementary to the Thomson scattering method in white light, which probes the CME evolution in the heliosphere at r>~2R_{sun}_, while the EUV dimming method tracks the CME launch in the corona. We compare the CME parameters obtained in white light with the LASCO/C2 coronagraph with those obtained from EUV dimming with the Atmospheric Imaging Assembly onboard the SDO for all identical events in both data sets. We investigate correlations between CME parameters, the relative timing with flare parameters, frequency occurrence distributions, and the energy partition between magnetic, thermal, nonthermal, and CME energies. CME energies are found to be systematically lower than the dissipated magnetic energies, which is consistent with a magnetic origin of CMEs.
- ID:
- ivo://CDS.VizieR/J/ApJ/885/49
- Title:
- Global energetics of solar flares. IX.
- Short Name:
- J/ApJ/885/49
- Date:
- 08 Dec 2021
- Publisher:
- CDS
- Description:
- A more accurate analytical solution of the vertical-current approximation nonlinear force-free field (VCA3-NLFFF) model is presented that includes, besides the radial (Br) and azimuthal (B{phi}) magnetic field components, a poloidal component (B_{theta}_/=0) as well. This new analytical solution is of second-order accuracy in the divergence-freeness condition and of third-order accuracy in the force-freeness condition. We reanalyze the sample of 173 GOES M- and X-class flares observed with the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). The new code reproduces helically twisted loops with a low winding number below the kink instability consistently, avoiding unstable, highly twisted structures of the Gold-Hoyle flux rope type. The magnetic energies agree within E_VCA3_/E_W_=0.99{+/-}0.21 with the Wiegelmann (W-NLFFF) code. The time evolution of the magnetic field reveals multiple, intermittent energy buildup and releases in most flares, contradicting both the Rosner-Vaiana model (with gradual energy storage in the corona) and the principle of timescale separation ({tau}flare<<{tau}storage) postulated in self-organized criticality models. The mean dissipated flare energy is found to amount to 7%{+/-}3% of the potential energy, or 60%{+/-}26% of the free energy, a result that can be used for predicting flare magnitudes based on the potential field of active regions.
- ID:
- ivo://CDS.VizieR/J/ApJ/814/1
- Title:
- GRB 120326A, 100418A & 100901A multi-wavelength obs.
- Short Name:
- J/ApJ/814/1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present multi-wavelength observations and modeling of gamma-ray bursts (GRBs) that exhibit a simultaneous re-brightening in their X-ray and optical light curves, and are also detected at radio wavelengths. We show that the re-brightening episodes can be modeled by injection of energy into the blastwave and that in all cases the energy injection rate falls within the theoretical bounds expected for a distribution of energy with ejecta Lorentz factor. Our measured values of the circumburst density, jet opening angle, and beaming-corrected kinetic energy are consistent with the distribution of these parameters for long-duration GRBs at both z~1 and z>~6, suggesting that the jet launching mechanism and environment of these events are similar to that of GRBs that do not have bumps in their light curves. However, events exhibiting re-brightening episodes have lower radiative efficiencies than average, suggesting that a majority of the kinetic energy of the outflow is carried by slow-moving ejecta, which is further supported by steep measured distributions of the ejecta energy as a function of Lorentz factor. We do not find evidence for reverse shocks over the energy injection period, implying that the onset of energy injection is a gentle process. We further show that GRBs exhibiting simultaneous X-ray and optical re-brightenings are likely the tail of a distribution of events with varying rates of energy injection, forming the most extreme events in their class. Future X-ray observations of GRB afterglows with Swift and its successors will thus likely discover several more such events, while radio follow-up and multi-wavelength modeling of similar events will unveil the role of energy injection in GRB afterglows.
- ID:
- ivo://CDS.VizieR/J/A+A/531/A12
- Title:
- Grid of Lya radiation transfer models
- Short Name:
- J/A+A/531/A12
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Ly{alpha} is a key diagnostic for numerous observations of distant star-forming galaxies, but detailed radiation transfer models are required for interpreting them. We provide an extensive grid of 3D radiation transfer models that simulate the Ly{alpha} and UV continuum radiation transfer in the interstellar medium of star-forming galaxies. We have improved our Monte Carlo MCLya code and used it to compute a grid of 6240 radiation transfer models for homogeneous spherical shells containing HI and dust surrounding a central source. The simulations cover a wide range of parameter space. We present the detailed predictions from our models including in particular the Ly{alpha} escape fraction fesc, the continuum attenuation, and detailed Ly{alpha} line profiles. The Ly{alpha} escape fraction is shown to depend strongly on dust content, but also on other parameters (HI column density and radial velocity). The predicted line profiles show a wide diversity of morphologies, ranging from broad absorption lines to emission lines with complex features. The results from our simulations are distributed in electronic format. Our models should be of use for interpreting observations from distant galaxies, for other simulations, and should also serve as an important basis for comparison for future, more refined, radiation transfer models.
- ID:
- ivo://CDS.VizieR/J/A+A/569/A124
- Title:
- GUViCS. Ultraviolet Source Catalogs
- Short Name:
- J/A+A/569/A124
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper we introduce the deepest and most extensive ultraviolet extragalactic source catalogs of the Virgo Cluster area to date. Archival and targeted GALEX imaging is compiled and combined to provide the deepest possible coverage over ~120deg^2^ in the NUV (lambda_eff_=2316{AA}) and ~40deg^2^ in the FUV (lambda_eff_=1539{AA}) between 180{deg}<=RA<=195{deg} and 0{deg}<=DE<=20{deg}. We measure the integrated photometry of 1770 extended UV sources of all galaxy types and use GALEX pipeline photometry for 1,230,855 point-like sources in the foreground, within, and behind the cluster. Extended source magnitudes are reliable to m_UV_~22, showing a ~0.01{sigma} difference from their asymptotic magnitudes. Point-like source magnitudes have a 1{sigma} standard deviation within ~0.2mag down to m_uv_~23. The point-like source catalog is cross-matched with large optical databases and surveys including the SDSS DR9 (>1 million Virgo Cluster sources), the Next Generation Virgo Cluster Survey (NGVS; >13 million Virgo Cluster sources), and the NED (~30,000 sources in the Virgo Cluster). We find that 69% of the entire UV point-like source catalog has a unique optical counterpart, 11% of which are stars and 0.01% (129) are Virgo cluster members that are neither in the VCC nor part of the bright CGCG galaxy catalog (i.e., m_pg_<14.5). These data are collected in three catalogs containing the UV extended sources, the UV point-like sources, and the most relevant optical parameters of UV-optically matched point-like sources for further studies from SDSS. The GUViCS catalogs provide a unique set of data for future work on UV and multiwavelength studies in the cluster and background environments.
- ID:
- ivo://CDS.VizieR/J/ApJ/744/44
- Title:
- H{alpha} and UV fluxes in nearby galaxies
- Short Name:
- J/ApJ/744/44
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We consider the effects of non-constant star formation histories (SFHs) on H{alpha} and GALEX far-ultraviolet (FUV) star formation rate (SFR) indicators. Under the assumption of a fully populated Chabrier initial mass function (IMF), we compare the distribution of H{alpha}-to-FUV flux ratios from ~1500 simple, periodic model SFHs with observations of 185 galaxies from the Spitzer Local Volume Legacy survey. We find a set of SFH models that are well matched to the data, such that more massive galaxies are best characterized by nearly constant SFHs, while low-mass systems experience burst amplitudes of ~30 (i.e., an increase in the SFR by a factor of 30 over the SFR during the inter-burst period), burst durations of tens of Myr, and periods of ~250 Myr; these SFHs are broadly consistent with the increased stochastic star formation expected in systems with lower SFRs. We analyze the predicted temporal evolution of galaxy stellar mass, R-band surface brightness, H{alpha}-derived SFR, and blue luminosity, and find that they provide a reasonable match to observed flux distributions. We find that our model SFHs are generally able to reproduce both the observed systematic decline and increased scatter in H{alpha}-to-FUV ratios toward low-mass systems, without invoking other physical mechanisms.
