arXiv subjects

Christophe Risacher

Publications and source records attributed to Christophe Risacher.

5 recordsLinked to original sources

arXiv2026

CII fine-structure line observations of the Sagittarius C Region in the Galaxy's Central Molecular Zone

Context. Sagittarius C (Sgr C) is a massive, relatively quiescent complex at the western edge of the Galaxy's Central Molecular Zone (CMZ). While the Sgr B2 region has been extensively studied, Sgr C has received comparatively less attention. Aims. We aim to characterize the kinematics and physical state of the gas in Sgr C using spatially and velocity-resolved [CII] 158 microns emission. This line traces the multi-phase interstellar medium, providing a crucial complement to molecular, infrared, and radio observations. Methods. We present a fully sampled 74x47 pc map of the [CII] line toward Sgr C, observed with SOFIA. The data feature a 0.55 pc spatial and 1 km/s spectral resolution. These observations are analyzed in conjunction with ancillary maps of the CO(2-1) transition and its isotopologues from the APEX telescope. Results. [CII] emission is widespread, showing a continuous structure extending from Sgr A to Sgr C with complex morphology. The bulk emission arises at negative radial velocities, consistent with Galactic rotation. The most prominent feature is the giant Sgr C HII region, where [CII] reveals an expanding, ring-like shell interpreted as a photo-dissociation region (PDR). Kinematic modelling yields an expansion velocity of 23 km/s and a dynamical age of about 0.13 Myr. Our analysis suggests that stellar winds from known massive stars are insufficient to power the observed expansion, pointing toward alternative drivers like a buried supernova. Finally, we find a striking spatial association between this shell and a non-thermal radio filament, indicating that the shell's expansion has triggered high-mass star formation at its edge.

astro-ph.GA↗

arXiv2020

4GREAT -- a four-color receiver for high-resolution airborne terahertz spectroscopy

4GREAT is an extension of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are co-aligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525 and 2490-2590 GHz, respectively. This paper presents the design and characterization of the instrument, and its in-flight performance. 4GREAT saw first light in June 2018, and has been offered to the interested SOFIA communities starting with observing cycle 6.

astro-ph.IM↗

arXiv2019

First astrophysical detection of the helium hydride ion (HeH$^+$)

During the dawn of chemistry when the temperature of the young Universe had fallen below $\sim$4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He$^{++}$ (54.5 eV) and He$^+$ (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH$^+$, by radiative association with protons (He + H$^+$ $\rightarrow$ HeH$^+$ + h$ν$). As recombination progressed, the destruction of HeH$^+$ (HeH$^+$ + H $\rightarrow$ He + H$_2^+$) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH$^+$ molecule has so far escaped unequivocal detection in interstellar space. In the laboratory, the ion was discovered as long ago as 1925, but only in the late seventies was the possibility that HeH$^+$ might exist in local astrophysical plasmas discussed. In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH$^+$ column densities: the hard radiation field from the central hot white dwarf creates overlapping Strömgren spheres, where HeH$^+$ is predicted to form, primarily by radiative association of He$^+$ and H. With the GREAT spectrometer onboard SOFIA, the HeH$^+$ rotational ground-state transition at $λ$149.1 $μ$m is now accessible. We report here its detection towards the planetary nebula NGC7027.

astro-ph.GA↗

arXiv2015

First supra-THz Heterodyne Array Receivers for Astronomy with the SOFIA Observatory

We present the upGREAT THz heterodyne arrays for far-infrared astronomy. The Low Frequency Array (LFA) is designed to cover the 1.9-2.5 THz range using 2x7-pixel waveguide-based HEB mixer arrays in a dual polarization configuration. The High Frequency Array (HFA) will perform observations of the [OI] line at ~4.745 THz using a 7-pixel waveguide-based HEB mixer array. This paper describes the common design for both arrays, cooled to 4.5 K using closed- cycle pulse tube technology. We then show the laboratory and telescope characterization of the first array with its 14 pixels (LFA), which culminated in the successful commissioning in May 2015 aboard the SOFIA airborne observatory observing the [CII] fine structure transition at 1.905 THz. This is the first successful demonstration of astronomical observations with a heterodyne focal plane array above 1 THz and is also the first time high- power closed-cycle coolers for temperatures below 4.5 K are operated on an airborne platform.

astro-ph.IM↗

arXiv2015

APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: IV. Mechanical and radiative feedback

During the embedded stage of star formation, bipolar molecular outflows and UV radiation from the protostar are important feedback processes. Our aim is to quantify the feedback, mechanical and radiative, for a large sample of low-mass sources. The outflow activity is compared to radiative feedback in the form of UV heating by the accreting protostar to search for correlations and evolutionary trends. Large-scale maps of 26 young stellar objects, which are part of the Herschel WISH key program are obtained using the CHAMP+ instrument on the APEX (12CO and 13CO 6-5), and the HARP-B instrument on the JCMT (12CO and 13CO 3-2). Maps are used to determine outflow parameters and envelope models are used to quantify the amount of UV-heated gas and its temperature from 13CO 6-5 observations. All sources in our sample show outflow activity and the outflow force, F_CO, is larger for Class 0 sources than for Class I sources, even if their luminosities are comparable. The outflowing gas typically extends to much greater distances than the power-law envelope and therefore influences the surrounding cloud material directly. Comparison of the CO 6-5 results with Herschel-HIFI H2O and PACS high-J CO lines, both tracing currently shocked gas, shows that the two components are linked, even though the transitions do not probe the same gas. The link does not extend down to CO 3-2. The conclusion is that CO 6-5 depends on the shock characteristics (density and velocity), whereas CO 3-2 is more sensitive to conditions in the surrounding environment (density). The radiative feedback is responsible for increasing the gas temperature by a factor of two, up to 30-50 K, on scales of a few thousand AU, particularly along the direction of the outflow. The mass of the UV heated gas exceeds the mass contained in the entrained outflow in the inner ~3000 AU and is therefore at least as important on small scales.

astro-ph.SR↗