A new method for estimating frequency-dependent core shifts in active galactic nucleus jets

We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.Comment: 8 pages, 6 figures, accepted for publication in MNRA

Connected components of spaces of Morse functions with fixed critical points

Let $M$ be a smooth closed orientable surface and $F=F_{p,q,r}$ be the space of Morse functions on $M$ having exactly $p$ critical points of local minima, $q\ge1$ saddle critical points, and $r$ critical points of local maxima, moreover all the points are fixed. Let $F_f$ be the connected component of a function $f\in F$ in $F$. By means of the winding number introduced by Reinhart (1960), a surjection $\pi_0(F)\to{\mathbb Z}^{p+r-1}$ is constructed. In particular, $|\pi_0(F)|=\infty$, and the Dehn twist about the boundary of any disk containing exactly two critical points, exactly one of which is a saddle point, does not preserve $F_f$. Let $\mathscr D$ be the group of orientation preserving diffeomorphisms of $M$ leaving fixed the critical points, ${\mathscr D}^0$ be the connected component of ${\rm id}_M$ in $\mathscr D$, and ${\mathscr D}_f\subset{\mathscr D}$ the set of diffeomorphisms preserving $F_f$. Let ${\mathscr H}_f$ be the subgroup of ${\mathscr D}_f$ generated by ${\mathscr D}^0$ and all diffeomorphisms $h\in{\mathscr D}$ which preserve some functions $f_1\in F_f$, and let ${\mathscr H}_f^{\rm abs}$ be its subgroup generated ${\mathscr D}^0$ and the Dehn twists about the components of level curves of functions $f_1\in F_f$. We prove that ${\mathscr H}_f^{\rm abs}\subsetneq{\mathscr D}_f$ if $q\ge2$, and construct an epimorphism ${\mathscr D}_f/{\mathscr H}_f^{\rm abs}\to{\mathbb Z}_2^{q-1}$, by means of the winding number. A finite polyhedral complex $K=K_{p,q,r}$ associated to the space $F$ is defined. An epimorphism $\mu:\pi_1(K)\to{\mathscr D}_f/{\mathscr H}_f$ and finite generating sets for the groups ${\mathscr D}_f/{\mathscr D}^0$ and ${\mathscr D}_f/{\mathscr H}_f$ in terms of the 2-skeleton of the complex $K$ are constructed.Comment: 12 pages with 2 figures, in Russian, to be published in Vestnik Moskov. Univ., a typo in theorem 1 is correcte

Age spread in Galactic star forming region W3 Main

We present near-infrared JHKs imaging as well as K-band multi-object spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT. We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral types between O5V and B4V have been found. Three massive Young Stellar Objects are identified by their emission line spectra and near-infrared excess. From our spectrophotometric analysis of the massive stars and the nature of their surrounding HII regions we derive the evolutionary sequence of W3 Main and we find an age spread of 2-3 Myr.Comment: 4 pages, 2 figures, To appear in conference proceedings of "370 years of Astronomy in Utrecht

Multiple episodes of star formation in the CN15/16/17 molecular complex

We have started a campaign to identify massive star clusters inside bright molecular bubbles towards the Galactic Center. The CN15/16/17 molecular complex is the first example of our study. The region is characterized by the presence of two young clusters, DB10 and DB11, visible in the NIR, an ultra-compact HII region identified in the radio, several young stellar objects visible in the MIR, a bright diffuse nebulosity at 8\mu m coming from PAHs and sub-mm continuum emission revealing the presence of cold dust. Given its position on the sky (l=0.58, b=-0.85) and its kinematic distance of ~7.5 kpc, the region was thought to be a very massive site of star formation in proximity of the CMZ. The cluster DB11 was estimated to be as massive as 10^4 M_sun. However the region's properties were known only through photometry and its kinematic distance was very uncertain given its location at the tangential point. We aimed at better characterizing the region and assess whether it could be a site of massive star formation located close to the Galactic Center. We have obtained NTT/SofI JHKs photometry and long slit K band spectroscopy of the brightest members. We have additionally collected data in the radio, sub-mm and mid infrared, resulting in a quite different picture of the region. We have confirmed the presence of massive early B type stars and have derived a spectro-photometric distance of ~1.2 kpc, much smaller than the kinematic distance. Adopting this distance we obtain clusters masses of M(DB10) ~ 170 M_sun and M(DB11) ~ 275 M_sun. This is consistent with the absence of any O star, confirmed by the excitation/ionization status of the nebula. No HeI diffuse emission is detected in our spectroscopic observations at 2.113\mu m, which would be expected if the region was hosting more massive stars. Radio continuum measurements are also consistent with the region hosting at most early B stars.Comment: Accepted for publication in Astronomy and Astrophysics. Fig. 1 and 3 presented in reduced resolutio