Prospects for IXPE and eXTP polarimetric archaeology of the reflection nebulae in the Galactic center

The X-ray polarization properties of the reflection nebulae in the Galactic center inform us about the direction of the illuminating source (through the polarization angle) and the cloud position along the line of sight (through the polarization degree). However, the detected polarization degree is expected to be lowered because the polarized emission of the clouds is mixed with the unpolarized diffuse emission that permeates the Galactic center region. In a real observation, also the morphological smearing of the source due to the point spread function and the unpolarized instrumental background contribute in diluting the polarization degree. So far, these effects have never been included in the estimation of the dilution. We evaluate the detectability of the X-ray polarization predicted for the MC2, Bridge-B2, G0.11-0.11, Sgr B2, Sgr C1, Sgr C2, and Sgr C3 molecular clouds with modern X-ray imaging polarimeters such as the Imaging X-ray Polarimetry Explorer (IXPE), which is expected to launch in 2021, and the Enhanced X-ray Timing and Polarimetry mission (eXTP), whose launch is scheduled for 2027. We perform realistic simulations of X-ray polarimetric observations considering (with the aid of Chandra maps and spectra) the spatial, spectral, and polarization properties of all the diffuse emission and background components in each region of interest. We find that in the 4.0–8.0 keV band, where the emission of the molecular clouds outshines the other components, the dilution of the polarization degree, including the contribution due to the morphological smearing of the source, ranges between ~19% and ~55%. We conclude that for some distance values reported in the literature, the diluted polarization degree of G0.11-0.11, Sgr B2, Bridge-B2, Bridge-E, Sgr C1, and Sgr C3 may be detectable in a 2 Ms long IXPE observations. With the same exposure time, and considering the whole range of possible distances reported in the literature, the enhanced capabilities of eXTP may allow detecting the 4.0–8.0 keV of all the targets considered here.

High-mass star formation in Orion possibly triggered by cloud–cloud collision. III. NGC 2068 and NGC 2071

Using the NANTEN2 Observatory, we carried out a molecular-line study of high-mass star forming regions with reflection nebulae, NGC 2068 and NGC 2071, in Orion in the $^{13}$CO($J = 2$–1) transition. The $^{13}$CO distribution shows that there are two velocity components at ${9.0}$ and ${10.5}\:$km$\:$s$^{-1}$. The blue-shifted component is in the northeast associated with NGC 2071, whereas the red-shifted component is in the southwest associated with NGC 2068. The total intensity distribution of the two clouds shows a gap of $\sim\!\! 1\:$pc, suggesting that they are detached at present. A detailed spatial comparison indicates that the two show complementary distributions. The blue-shifted component lies toward an intensity depression to the northwest of the red-shifted component, where we find that a displacement of ${0.8}\:$pc makes the two clouds fit well with each other. Furthermore, a new simulation of non-frontal collisions shows that observations from $60^\circ$ off the collisional axis agreed well with the velocity structure in this region. On the basis of these results, we hypothesize that the two components collided with each other at a projected relative velocity of ${3.0}\:$km$\:$s$^{-1}$. The timescale of the collision is estimated to be ${0.3}\:$Myr for an assumed axis of the relative motion $60^\circ$ off the line of sight. We assume that the two most massive early B-type stars in the cloud, illuminating stars of the two reflection nebulae, were formed by collisional triggering at the interfaces between the two clouds. Given the other young high-mass star-forming regions, namely, M 42, M 43, and NGC 2024 (Fukui et al. 2018a, ApJ, 859, 166; Ohama et al. 2017, arXiv:1706.05652), it seems possible that collisional triggering has been independently working to form O-type and early B-type stars in Orion in the last Myr over a projected distance of ∼80 pc.

Two epoch spectro-imagery of FS Tau B outflow system

Context. Herbig–Haro (HH) flows exhibit a large variety of morphological and kinematical structures such as bow shocks, Mach disks, and deflection shocks. Both proper motion (PM) and radial velocity investigations are essential to understand the physical nature of such structures. Aims. We investigate the kinematics and PM of spectrally separated structures in the FS Tau B HH flow. Collating these data makes it possible to understand the origin of these structures and to explain the unusual behavior of the jet. Besides, the study of emission profiles in the associated reflection nebulae allows us to consider the source of the outflow both from edge-on and pole-on points of view. Methods. We present the data obtained with the 6 m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences using the SCORPIO multimode focal reducer with a scanning Fabry–Perot interferometer. Two epochs of the observations of the FS Tau B region in Hα emission (2001 and 2012) allowed us to measure the PM of the spectrally separated inner structures of the jet. Results. In addition to already known emission structures in the FS Tau B system, we discover new features in the extended part of the jet and in the counter-jet. Moreover, we reveal a new HH knot in the HH 276 independent outflow system and point out its presumable source. In the terminal working surface of the jet, structures with different radial velocities have PMs of the same value. This result can be interpreted as the direct observation of bow-shock and Mach disk regions. A bar-like structure, located southwest from the source demonstrates zero PM and can be considered as one more example of deflection shock. An analysis of Hα profiles in the reflection nebulae R1 and R3 indicates the uniqueness of this object, which can be studied in pole-on and edge-on directions simultaneously.