arXiv subjects

José Franco

Publications and source records attributed to José Franco.

7 recordsLinked to original sources

arXiv2026

Synthetic polarization observations of magnetized pillars in HII regions: Assessing the reliability of the Davis-Chandrasekhar-Fermi method

We investigated the morphology and strength of magnetic fields in pillar-shaped structures at the boundaries of HII regions by combining three-dimensional radiation-magnetohydrodynamic (R-MHD) simulations with synthetic polarimetric and molecular-line observations. Our analysis focuses on a self-consistently formed pillar as a proof of concept to test the Davis-Chandrasekhar-Fermi (DCF) method under externally driven conditions. The pillar arises as an ionization front compresses a dense clump, producing a magnetically aligned, elongated structure whose morphology and field configuration resemble systems such as the pillars in M16. Synthetic 850 μm dust-polarization maps reproduce the pillar's large-scale magnetic-field morphology, confirming polarimetry as a reliable tracer of magnetic-field geometry. To evaluate DCF-based methods, we extract local density and velocity dispersion self-consistently from synthetic 13CO observations and measure polarization-angle dispersion using single-Gaussian fits to the synthetic polarization-angle distributions. We find that DCF-based methods systematically overestimate the intrinsic plane-of-sky magnetic-field strength by average factors of ~7 for the classical DCF method and ~5 for the modified Skalidis & Tassis formulation. This overestimation is already present in the full-pillar measurement and is not removed by applying polarimetric S/N cuts or by excluding the dynamically complex head. We attribute the discrepancy to external compression by the expanding H II region, which organizes the magnetic field on pillar scales while driving non-thermal gas motions. Consequently, the measured velocity and polarization-angle dispersions no longer trace the same turbulence-driven perturbation field assumed by DCF. Our results highlight the need for caution when applying DCF-based analyses to pillars or other externally compressed structures.

astro-ph.GA↗

arXiv2025

Pressure Regulated Formation of Molecular Clouds and Stars: The case of the Milky Way

We present a steady-state analytical model for pressure-regulated formation of molecular clouds (MC) and stars (SF) in gaseous galactic disks and apply it to the Milky Way (MW). MC formation depends on midplane interstellar pressure $P_{\text{ISM}}$ and metallicity $Z$, and for galactocentric distances $R\gtrsim5$ kpc, $P_{\text{ISM}}(R)$ scales approximately linearly with molecular gas surface density $Σ_{\rm mol}(R)$. The molecularization of the cold neutral medium (CNM) is due to the opacity of small dust grains that protect the center of the cloud from dissociating radiation when the column density is $Σ_d\geq 5\ (Z_\odot/Z)M_\odot\text{ pc}^{-2}$. The H$_2$ formation rate per hydrogen atom is $F\sim10^{-15}(P_{\text{ISM}}/P_\odot)T_{100}^{-1/2}\text{s}^{-1}$, and the corresponding formation rate per unit area is $\dotΣ^{+}_{\rm mol}\sim 5\times10^{-2}\left(P_{\text{ISM}}/{P_\odot}\right)T_{100}^{-1/2}M_\odot~\text{kpc}^{-2}~\text{yr}^{-1}$, where $P_\odot$ is the pressure at the solar circle and $T_{100}=T/100\text{ K}$ is the temperature of the cloud. In equilibrium, this equals the molecular gas destruction rate $\dotΣ^{-}_{\rm mol}$ due to SF. Self-gravity sets in when the column density of a cloud reaches $Σ_{\rm sg}=Σ_{\rm sg,\odot}(P_{\text{ISM}}/P_\odot)^{1/2}$, with $Σ_{\rm sg,\odot}\sim30\ M_\odot\ \text{pc}^{-2}$. Given the distribution of $P_{\text{ISM}}(R)$ and $Z(R)$ in the MW, the SF process at $5\lesssim R\lesssim11$ kpc follows a two-step track: first, MCs form from CNM gas and then they form stars when self-gravity sets in. The resulting SFR surface density is $Σ_\text{SFR}(R)\approx (1.6-4)\times10^{-3}\left(P_{\text{ISM}}/P_\odot\right)\ \text{M}_\odot~\text {kpc}^{-2}\text{yr}^{-1}$ with an average final SF efficiency of $ε_{\rm sf}\sim (3-8)\times 10^{-2}$.

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arXiv2019

Ultracompact H II Regions with Extended Emission: The Complete View

In this paper we present the results of a morphological study performed on a sample of 28 ultracompact \hiirs~located near extended free-free emission, using radio continuum observations at 3.6~cm with the C and D VLA~configurations, with the aim of determining a direct connection between them. By using previously published observations in B and D VLA~configurations, we compiled a final catalogue of 21 ultracompact \hiirs ~directly connected with the surrounding extended emission. The observed morphology of most of the ultracompact \hiirs~in radio continuum emission is irregular (single or multi-peaked sources) and resembles a classical bubble structure in the Galactic plane with well-defined cometary arcs. Radio continuum images superimposed on colour composite \textit{Spitzer} images reinforce the assignations of direct connection by the spatial coincidence between the ultracompact components and regions of saturated 24~\micron~emission. We also find that the presence of extended emission may be crucial to understand the observed infrared-excess because an underestimation of ionizing Lyman photons was considered in previous works.

astro-ph.GA↗

arXiv2019

Structure and Expansion Law of HII Regions in structured Molecular Clouds

We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H\,{\normalsize II} regions in turbulent, magnetised, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionising radiation eventually stops the accretion (through filaments) toward the massive star-forming regions. The new over-pressured H\,{\normalsize II} regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H\,{\normalsize II} regions expand in a hybrid manner: they virtually do not expand toward the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionised regions grow anisotropically, and the ionising stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H\,{\normalsize II}-region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H\,{\normalsize II} regions, producing a net expansion law ($R \propto t^α$, with $α$ in the range of 0.93-1.47 and a mean value of $1.2 \pm 0.17$) that differs from any of those of the standard models.

astro-ph.GA↗

arXiv2019

TOROS Optical follow-up of the Advanced LIGO-VIRGO O2 second observational campaign

We present the results of the optical follow-up, conducted by the TOROS collaboration, of gravitational wave events detected during the Advanced LIGO-Virgo second observing run (Nov 2016 -- Aug 2017). Given the limited field of view ($\sim100\arcmin$) of our observational instrumentation we targeted galaxies within the area of high localization probability that were observable from our sites. We analyzed the observations using difference imaging, followed by a Random Forest algorithm to discriminate between real and bogus transients. For all three events that we respond to, except GW170817, we did not find any bona fide optical transient that was plausibly linked with the observed gravitational wave event. Our observations were conducted using telescopes at Estación Astrof\'ısica de Bosque Alegre, Cerro Tololo Inter-American Observatory, and the Dr. Cristina V. Torres Memorial Astronomical Observatory. Our results are consistent with the LIGO-Virgo detections of a binary black hole merger (GW170104) for which no electromagnetic counterparts were expected, as well as a binary neutron star merger (GW170817) for which an optical transient was found as expected.

astro-ph.HE↗

arXiv2009

The Synoptic All-Sky Infrared (SASIR) Survey

We are proposing to conduct a multicolor, synoptic infrared (IR) imaging survey of the Northern sky with a new, dedicated 6.5-meter telescope at San Pedro Mártir (SPM) Observatory. This initiative is being developed in partnership with astronomy institutions in Mexico and the University of California. The 4-year, dedicated survey, planned to begin in 2017, will reach more than 100 times deeper than 2MASS. The Synoptic All-Sky Infrared (SASIR) Survey will reveal the missing sample of faint red dwarf stars in the local solar neighborhood, and the unprecedented sensitivity over such a wide field will result in the discovery of thousands of z ~ 7 quasars (and reaching to z > 10), allowing detailed study (in concert with JWST and Giant Segmented Mirror Telescopes) of the timing and the origin(s) of reionization. As a time-domain survey, SASIR will reveal the dynamic infrared universe, opening new phase space for discovery. Synoptic observations of over 10^6 supernovae and variable stars will provide better distance measures than optical studies alone. SASIR also provides significant synergy with other major Astro2010 facilities, improving the overall scientific return of community investments. Compared to optical-only measurements, IR colors vastly improve photometric redshifts to z ~ 4, enhancing dark energy and dark matter surveys based on weak lensing and baryon oscillations. The wide field and ToO capabilities will enable a connection of the gravitational wave and neutrino universe - with events otherwise poorly localized on the sky - to transient electromagnetic phenomena.

astro-ph.IM↗

arXiv2008

On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis

Here we address the question whether the ionized shells associated with giant HII regions can be progenitors of the larger HI shell-like objects found in the Milky Way and other spiral and dwarf irregular galaxies. We use for our analysis a sample of 12 HII shells presented recently by Relaño et al. (2005, 2007). We calculate the evolutionary tracks that these shells would have if their expansion is driven by multiple supernovae explosions from the parental stellar clusters. We find, contrary to Relaño et al. (2007), that the evolutionary tracks of their sample HII shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. We conclude that HII shells found inside giant HII regions may represent the progenitors of small or intermediate HI shells, however they cannot evolve into the largest HI objects unless, aside from the multiple supernovae explosions, an additional energy source contributes to their expansion.

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