- Published on 11 September 2018
In section 6. Interstellar and circumstellar matter
Anatomy of the massive star-forming region S106. The OI 63 micron line observed with GREAT/SOFIA as a versatile diagnostic tool for the evolution of massive stars
The star-forming region S106 has been an object of intense interest for decades as a model region for studying massive star formation. These new spectroscopic observations, performed with GREAT/SOFIA have superb spatial (3 arcsec stepping, about 500 AU) and velocity (about 0.04 km/s) resolutions. They are supplemented with IRAM mm and archival VLA cm and Herschel IR imaging to produce a comprehensive, virtually tomographic, picture of the region. Particularly lovely is the association of different parts of the [O I] profile with structures in the cm radio imaging. The way in which the [O I] precisely traces the ionised gas (from cm observations) in the low velocity interval of the line profiles. The paper highlights how high spectral resolution and multiple tracers provide the three-dimensional ionization, density, and velocity structure, even distinguishing between shock and radiative excitations. This paper serves as a model analysis for future observational programs on spatially resolved star forming regions.
- Published on 04 September 2018
In section 8. Stellar atmospheres
Spatially resolved spectroscopy across stellar surfaces. III. Photospheric FeI lines across HD189733A (K1 V)
This paper presents spatially resolved, high spectral-resolution spectroscopy with the ESO HARPS, of photospheric Fe I lines over the stellar surface of the cool planet-hosting K1 V star, HD 189733A. This is possible during an exoplanet transit as successive portions of the stellar surface become hidden, and differential spectroscopy between these various transit phases provides spectra of small surface segments temporarily hidden behind the planet. This provides spectral line profiles that are free from rotational broadening, and so their gradual change from the center towards the stellar limb reflects the fine structure in the stellar atmosphere. These observations are compared to 3D hydrodynamic models to explore fine structure and 3D line formation in the atmosphere of this star. This detailed understanding is important to enable future searches for Earth-analogue exoplanets around K-type stars, where the more tranquilsurface granulation and lower ensuing micro-variability may allow suchdetections. Fig. for the HL in Author to Editor May 24.
- Published on 30 August 2018
In section 2. Astrophysical processes
PSR B0943+10: low-frequency study of subpulse periodicity in the Bright mode with LOFAR
The radio pulsar B0943+10 is peculiar as it switches abruptly between (radio) bright and quiet modes. These mode changes occur simultaneously in the radio and X-ray bands. Slow variations of the emission properties in the bright mode are observed (drifting subpulses), providing a way to study the pulsar magnetospheric configuration. Drifting subpulses are usually explained in terms of gradient of the accelerating potential in the polar gap, which causes the (fixed) emitting region to rotate around the magnetic axis (carousel model). Using wideband LOFAR data, the author provides a quantitative explanation of the observed frequency-dependent drift phase delay within the carousel model.
- Published on 21 August 2018
In section 10. Planets and planetary systems
Energy conversion in cometary atmospheres. Hybrid modeling of 67P/Churyumov-Gerasimenko
Solar wind cavity modeling with cometary ion production and solar wind penetration is used to determine the local dynamo activity and energization of the particles by electromagnetic interaction with the background plasma. The model results are strikingly similar to the 67P observations.
- Published on 07 August 2018
In section 7. Stellar structure and evolution
The evolving jet spectrum of the neutron star X-ray binary Aql X-1 in transitional states during its 2016 outburst
Aql X-1 is the most prolific neutron star X-ray binary transient. Despite there being more than 30 outbursts observed so far, the simultaneous coverage in X-rays and radio band is poor. The 2016 outburst from Aql X-1 was observed seven times in the radio (ATCA) and mm (ALMA) bands, together with near-daily observations at X-ray and optical frequencies by Diaz Trigo et al. (2017). For the first time it was possible to see the rise to the outburst peak and the decay after at all frequencies. Radio and mm observations are consistent with the presence of a jet. The synchrotron emission peak shifts from ~100 GHz to ~5 GHz during the rising phase and back to ~30-100 GHz during the decay. This is the first time that these frequency shifts have been observed in a transient system containing a neutron star. A similar behaviour has been revealed in several black hole transients (even if for these sources a correlation of the radio peak with the X-ray spectrum holds, which is not detected in Aql X-1). This should indicate that physical processes at play for the jet formation depend only mildly on black hole mass or spin, or presence of a hard surface or the neutron star magnetic field, and should rely instead on the accretion disc and its corona properties.
- Published on 20 July 2018
In section 5. Galactic structure
The vertical force in the solar neighbourhood using red clump stars in TGAS and RAVE
Many estimations of the volumic mass density of the Milky Way in the solar neighborhood have already been made from a detailed census of the vertical dynamics of the stars, in order to derive the local dark matter density. Now with the first GAIA data release (TGAS), positions of stars and their proper motions have increased by almost a factor of two in precision, and most importantly, the sample of stars involved is nearly 20 times larger. It is, then, timely to revisit these estimations. The authors have studied the kinematics of red clump stars in the solar neighborhood by combining data from TGAS and RAVE. They applied the axisymmetric Jeans equations to subsets of stars representing the thin and thick disks to determine the local distribution of mass near the disk of our Galaxy. They determine two values of the z-force, Kz, for the thin and thick disks at 1.5 kpc away from the Galactic plane, assuming scale heights of 0.28 kpc and 1.12 kpc respectively. These measurements can be translated into a local dark matter density of 0.018 +/- 0.002 M_sol/pc^3, which is in very good agreement with previous estimations. However, the statistical error above is an underestimation of the actual uncertainty, which is dominated by systematic errors due to various assumptions (equilibrium or breathing of the stars, the nature of the various components, the interstellar mass, etc). A slight error of only 10% on the scale height of the thin disk leads to a 30% change in the value of the dark matter density. Future Gaia data releases will characterize the different stellar components in the Galaxy better and more precisely, and thus allow more precise determinations of the density.