421 research outputs found
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 be a smooth closed orientable surface and be the space
of Morse functions on having exactly critical points of local minima,
saddle critical points, and critical points of local maxima,
moreover all the points are fixed. Let be the connected component of a
function in . By means of the winding number introduced by Reinhart
(1960), a surjection is constructed. In
particular, , 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 . Let be the group of orientation
preserving diffeomorphisms of leaving fixed the critical points, be the connected component of in , and
the set of diffeomorphisms preserving
. Let be the subgroup of generated by
and all diffeomorphisms which preserve some
functions , and let be its subgroup
generated and the Dehn twists about the components of level
curves of functions . We prove that if , and construct an epimorphism
, by means of
the winding number. A finite polyhedral complex associated to the
space is defined. An epimorphism and finite generating sets for the groups
and in terms of the 2-skeleton of the complex
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
- …