57 research outputs found
On the alignment of PNe and local magnetic field at the galactic centre: MHD numerical simulations
For the past decade observations of the alignement of PNe symmetries with
respect to the galactic disk have led to conflicting results. Recently
observational evidence for alignment between PNe and local interstellar
magnetic fields in the central part of the Galaxy () has been
found. We studied the role of the interstellar magnetic field on the dynamical
evolution of a PN by means of an analytical model and from 3D MHD numerical
simulations. We test under what conditions typical ejecta would have their
dynamics severely modified by an interstellar magnetic field. We found that
uniform fields of G are required in order to be dynamically dominant.
This is found to occur only at later evolutionary stages, therefore being
unable to change the general morphology of the nebula. However, the symmetry
axis of bipolar and elliptical nebulae end up aligned to the external field.
This result can explain why different samples of PNe result in different
conclusions regarding the alignment of PNe. Objects located at high galactic
latitudes, or at large radii, should present no preferential alignment with
respect to the galactic plane. PNe located at the galactic centre and low
latitudes would, on the other hand, be preferentiably aligned to the disk.
Finally, we present synthetic polarization maps of the nebulae to show that the
polarization vectors, as well as the field lines at the expanding shell, are
not uniform even in the strongly magnetized case, indicating that polarization
maps of nebulae are not adequate in probing the orientation, or intensity, of
the dominant external field.Comment: to appear in MNRA
Revisiting the mass of open clusters with \emph{Gaia} data
The publication of the \emph{Gaia} catalogue and improvements in methods to
determine memberships and fundamental parameters of open clusters has led to
major advances in recent years. However, important parameters such as the
masses of these objects, although being studied mostly in some isolated cases,
have not been addressed in large homogeneous samples based on \emph{Gaia} data,
taking into account details such as binary fractions. Consequently, relevant
aspects such as the existence of mass segregation were not adequately studied.
Within this context, in this work, we introduce a new method to determine
individual stellar masses, including an estimation for the ones in binary
systems. This method allows us to study the mass of open clusters, as well as
the mass functions of the binary star populations. We validate the method and
its efficiency and characterize uncertainties using a grid of synthetic
clusters with predetermined parameters. We highlight the application of the
method to the Pleiades cluster, showing that the results obtained agree with
the current consensus in the literature as well as recent \emph{Gaia} data. We
then applied the procedure to a sample of 773 open clusters with fundamental
parameters determined using \emph{Gaia Early Data Release 3 (eDR3)} data,
obtaining their masses. Subsequently, we investigated the relation between the
masses and other fundamental parameters of the clusters. Among the results, we
found no significant evidence that clusters in our sample lose and segregate
mass with age.Comment: 26 pages, 17 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
3-D Photoionization Structure and Distances of Planetary Nebulae III. NGC 6781
Continuing our series of papers on the three-dimensional (3-D) structures of
and accurate distances to Planetary Nebulae (PNe), we present our study of the
planetary nebula NGC6781. For this object we construct a 3-D photoionization
model and, using the constraints provided by observational data from the
literature we determine the detailed 3-D structure of the nebula, the physical
parameters of the ionizing source and the first precise distance. The procedure
consists in simultaneously fitting all the observed emission line morphologies,
integrated intensities and the 2-D density map from the [SII] line ratios to
the parameters generated by the model, and in an iterative way obtain the best
fit for the central star parameters and the distance to NGC6781, obtaining
values of 950+-143pc and 385 Lsun for the distance and luminosity of the
central star respectively. Using theoretical evolutionary tracks of
intermediate and low mass stars, we derive the mass of the central star of
NGC6781 and its progenitor to be 0.60+-0.03 Msun and 1.5+-0.5 Msun
respectively.Comment: 16 pp, 6 figues, 2 tables, submitted to the Ap
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