- ID:
- ivo://CDS.VizieR/J/A+A/550/A106
- Title:
- Low-velocity shocks models
- Short Name:
- J/A+A/550/A106
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated. We establish and quantify the diagnostics of turbulence dissipation in mildly irradiated diffuse gas in the specific context of shock structures. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G_0_=1) magnetised shocks in diffuse media (n_H_=10^2^ to 10^4^cm^-3^) at low- to moderate velocities (from 3 to 40km/s). The formation of some molecules relies on endoergic reactions. Their abundances in J shocks are enhanced by several orders of magnitude for shock velocities as low as 7km/s. Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30km/s, where H_2_ dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan's Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5km/s), dissipation is due to ion-neutral friction and it powers H_2_ low-energy transitions and atomic lines. In moderate velocity shocks (20km/s and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C^+^ emission may trace shocks in UV irradiated gas where C^+^ is the dominant carbon species.} Low- and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/374/265
- Title:
- LTE spectrum synthesis in magnetic atmospheres
- Short Name:
- J/A+A/374/265
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Files table4.dat contain calculated Stokes IQUV local line profiles of Fe II 4923.9 shown in Fig. 4 of this paper. Profile sets are provided for 0.1, 5 and 20kG magnetic fields, with vector orientation {psi}=40{deg}, {phi}=0{deg}, disc centre and eps_fe_=4.6.
- ID:
- ivo://CDS.VizieR/J/MNRAS/475/5144
- Title:
- Magnetic early B-type stars. I.
- Short Name:
- J/MNRAS/475/5144
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The rotational and magnetic properties of many magnetic hot stars are poorly characterized, therefore the MiMeS and BinaMIcS collaborations have collected extensive high-dispersion spectropolarimetric datasets of these targets. We present longitudinal magnetic field measurements <B_Z_> for 52 early B-type stars (B5 to B0), with which we attempt to determine their rotational periods Prot. Supplemented with high-resolution spectroscopy, low-resolution DAO circular spectropolarimetry, and archival Hipparcos photometry, we determined Prot for 10 stars, leaving only 5 stars for which Prot could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of Prot is very similar to that observed for the cooler Ap/Bp stars. We also measured vsini and v_mac_ for all stars. Comparison to non-magnetic stars shows that vsini is much lower for magnetic stars, an expected consequence of magnetic braking. We also find evidence that v_mac_ is lower for magnetic stars. LSD profiles extracted using single-element masks revealed widespread, systematic discrepancies in <B_Z_> between different elements: this effect is apparent only for chemically peculiar stars, suggesting it is a consequence of chemical spots. Sinusoidal fits to H line <B_Z_> measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in 6 stars for which this has not previously been reported; however, in all 6 cases the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.
- ID:
- ivo://CDS.VizieR/J/MNRAS/390/545
- Title:
- Magnetic field and velocity of early M dwarfs
- Short Name:
- J/MNRAS/390/545
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0-M3), that is above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarized profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large-scale magnetic topologies of six early M dwarfs. We find that early-M stars preferentially host large-scale fields with dominantly toroidal and non-axisymmetric poloidal configurations, along with significant differential rotation (and long-term variability); only the lowest-mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large-scale field resembling those found in mid-M dwarfs.
- ID:
- ivo://CDS.VizieR/J/MNRAS/390/567
- Title:
- Magnetic field and velocity of mid M dwarfs
- Short Name:
- J/MNRAS/390/567
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present in this paper, the first results of a spectropolarimetric analysis of a small sample (~20) of active stars ranging from spectral type M0 to M8, which are either fully convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully convective stars. This paper focuses on ve stars of spectral type ~M4, i.e. with masses close to the full convection threshold (0.35M_{sun}_), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarized profiles. We find that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by 1yr; we find the magnetic topologies to be globally stable on this time-scale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field.
- ID:
- ivo://CDS.VizieR/J/other/AstBu/62.244
- Title:
- Magnetic field in CP stars
- Short Name:
- J/other/AstBu/62
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The dependence of the degree of anomaly of parameter Z of Geneva photometry (Z0=Z_CP_-Z_norm_) on the average surface magnetic field Bs is analyzed. The Z0 value is proportional to the degree of anomaly of chemical composition. It was found that Bs-->0 corresponds Z0-->-0.010/-0.015, i.e., part of CP stars are virtually devoid of magnetic field, but exhibit chemical anomalies. This effect may be due to selection whereby only objects with strong chemical anomalies are classified as CP stars, thereby producing a deficit of stars with relatively weak anomalies. Moreover, CP stars have other sources of stabilization of their atmospheres besides the magnetic field, e.g., slow rotation. Formulas relating Z0 to Bs are derived.
- ID:
- ivo://CDS.VizieR/J/ApJ/688/1029
- Title:
- Magnetic field in the SMC
- Short Name:
- J/ApJ/688/1029
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of the magnetic field of the Small Magellanic Cloud (SMC), carried out using radio Faraday rotation and optical starlight polarization data. Consistent negative rotation measures (RMs) across the SMC indicate that the line-of-sight magnetic field is directed uniformly away from us with a strength 0.19+/-0.06{mu}G. Applying the Chandrasekhar-Fermi method to starlight polarization data yields an ordered magnetic field in the plane of the sky of strength 1.6+/-0.4uG oriented at a position angle 4+/-12{deg}, measured counterclockwise from the great circle on the sky joining the SMC to the Large Magellanic Cloud (LMC). We construct a three-dimensional magnetic field model of the SMC, under the assumption that the RMs and starlight polarization probe the same underlying large-scale field. further test the pan-Magellanic field hypothesis.
- ID:
- ivo://CDS.VizieR/J/A+A/411/565
- Title:
- Magnetic field in V2052 Oph
- Short Name:
- J/A+A/411/565
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- V2052 Oph is a {beta} Cep star with vsini~60km/s. The behavior of its stellar wind was studied in the ultraviolet wavelength region with the IUE satellite. It revealed periodic variations in the equivalent widths (EW) of the resonance lines of wind-sensitive ions with a period of 3.638833d, which is identified as the rotational period. These variations are typical of magnetic stars. Therefore time-resolved circular spectropolarimetric observations were obtained with the Musicos echelle spectropolarimeter at the 2-m Telescope Bernard Lyot (TBL) to search for a magnetic field in the star. Stokes V patterns were observed, the inclination and magnetic angles were derived and a value was determined for the polar magnetic field (250+/-190G) using an oblique rotator dipole model. The spectroscopic information was used to search for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves. Multiperiodicity was found, corresponding to radial (f1=7.15c/d) and non-radial (f2=6.82c/d, l=3 or 4) pulsation modes. The rotation period is also detected in rotationally modulated observables because of the magnetic poles passing through the observer's line of sight. We determined the stellar parameters of the star, which was found to be chemically peculiar, in particular He-enriched. This makes V2052 Oph the first magnetic He-strong early B star with known pulsational properties.
- ID:
- ivo://CDS.VizieR/J/ApJ/873/L5
- Title:
- Magnetic field measurements of Ap/Bp stars
- Short Name:
- J/ApJ/873/L5
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on magnetic field measurements of 157 chemically peculiar A/B stars (Ap/Bp) based on resolved, magnetically split absorption lines present in H-band spectra provided by the Sloan Digital Sky Survey (SDSS)/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. These stars represent the extreme magnetic end of a still-growing sample of >900 Ap/Bp stars selected among the APOGEE telluric standard stars as those with CeIII absorption lines and/or literature Ap/Bp classifications. The lines most frequently resolved into their split components for these stars in the H-band pertain primarily pertain to CeIII, CrII, FeI, MnII, SiI, and CaII, in addition to one or more unidentified ions. Using mean magnetic field modulus (<B>) estimates for transitions with known Lande factors, we estimate effective Lande factors for 5 CeIII lines and 15 unknown lines and proceed to measure <B> of 157 stars, only 3 of which have previous literature estimates of <B>. This 183% increase in the number of Ap/Bp stars for which <B> has been measured is a result of the large number of stars observed by SDSS/APOGEE, extension of high-resolution Ap/Bp star observations to fainter magnitudes, and the advantages of long wavelengths for resolving magnetically split lines. With <B>~25kG, the star 2MASS J02563098+4534239 is currently the most magnetic star of the SDSS/APOGEE sample. Effective Lande factors, representative line profiles, and magnetic field moduli are presented. The validity of the results is supported using optical, high-resolution, follow-up spectra for 29 of the stars.
- ID:
- ivo://CDS.VizieR/J/MNRAS/510/6085
- Title:
- Magnetic field of Taurus/B211
- Short Name:
- J/MNRAS/510/6085
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Optical and infrared polarization mapping and recent Planck observations of the filamentary cloud L1495 in Taurus show that the large-scale magnetic field is approximately perpendicular to the long axis of the cloud. We use the HAWC+ polarimeter on SOFIA to probe the complex magnetic field in the B211 part of the cloud. Our results reveal a dispersion of polarization angles of 36{deg}, about five times that measured on a larger scale by Planck. Applying the Davis-Chandrasekhar-Fermi (DCF) method with velocity information obtained from IRAM 30m C^18^O(1-0) observations, we find two distinct sub-regions with magnetic field strengths differing by more than a factor 3. The quieter sub-region is magnetically critical and sub-Alfvenic; the field is comparable to the average field measured in molecular clumps based on Zeeman observations. The more chaotic, super-Alfvenic sub-region shows at least three velocity components, indicating interaction among multiple substructures. Its field is much less than the average Zeeman field in molecular clumps, suggesting that the DCF value of the field there may be an underestimate. Numerical simulation of filamentary cloud formation shows that filamentary substructures can strongly perturb the magnetic field. DCF and true field values in the simulation are compared. Pre-stellar cores are observed in B211 and are seen in our simulation. The appendices give a derivation of the standard DCF method that allows for a dispersion in polarization angles that is not small, present an alternate derivation of the structure function version of the DCF method, and treat fragmentation of filaments.
