arXiv subjects

M. Rubio

Publications and source records attributed to M. Rubio.

At least 19 recordsLinked to original sources

arXiv2026

Interface pinch-off in the presence of a soluble surfactant

We study numerically and experimentally the breakup of a pendant droplet loaded with a soluble surfactant. We consider the limit in which surfactant sorption is limited only by diffusion. Surfactant transfer toward the interface is enhanced by convection. As a consequence, diffusion does not constitute a significant barrier over most of the breakup, and surfactant sorption maintains the surface tension practically constant across the interface. Diffusion hinders the surfactant sorption only very close to the interface pinch-off. The droplet shape in the diffusion-limited model deviates significantly from that in the insoluble case over most of the breakup. In the insoluble case, the droplet shape is affected by surfactant depletion, which leads to a local increase in surface tension and Marangoni stress. The dynamics of a millimeter-sized droplet loaded with Surfynol 465 agree remarkably well with predictions from the diffusion-limited model, without any parameter fitting, down to pinching times of the order of $10-20$ $\mu$s. Sodium dodecyl sulfate (SDS) produces essentially the same effects as those for Surfynol 465. Therefore, both Surfynol 465 and SDS maintain a practically constant surface tension throughout most of the droplet breakup. Slow-kinetics surfactants, such as Triton X-100, differ significantly from Surfynol 465 and SDS. The most evident effect of the surfactant adsorption energy barrier is the shortening of the filament that bridges the upper meniscus and the detached lower drop. Comparing the filament length to that of a clean interface with the same surface tension allows one to evaluate the rate of surfactant adsorption.

physics.flu-dyn↗

arXiv2025

LMC+: Large-scale mapping of [CII] and [OIII] in the LMC molecular ridge, I. Dataset and line ratio analyses

The fundamental process of star formation in galaxies involves the interplay between the fueling of star formation via molecular gas and the feedback from recently formed massive stars. This process, by which galaxies evolve, is also closely connected to the intrinsic properties of the interstellar medium (ISM). To study the role that different molecular and atomic phases of the ISM play in star formation, and to characterize their physical conditions, we zoom into our nearest neighboring galaxy, the Large Magellanic Cloud (LMC; 50 kpc). The LMC offers a view of the ISM and star formation conditions in a low metallicity environment similar to, in that regard, the epoch of the peak of star formation in the earlier universe. We present an unprecedentedly detailed analysis of a well-known star-forming regions (SFRs) at a spatial resolution of a few pc. We mapped a 610pcx260pc region in the LMC molecular ridge in [CII] and the [OIII] using the FIFI-LS instrument on the SOFIA telescope. We compare the data with the distribution of the CO (2-1) emission from ALMA, the modeled TIR luminosity as well as Spitzer/MIPS continuum and Halpha. We also provide a detailed description of the observing strategy and the data reduction. We find that [CII] and [OIII] emission is associated with the SFRs in the molecular ridge, but also extends throughout the mapped region, not obviously associated with ongoing star formation. The CO emission is clumpier than the [CII] emission and we find plentiful [CII] present where there is little CO emission, possibly holding important implications for CO-dark gas. We find a clear trend of the [CII]/TIR ratio decreasing with increasing TIR. This suggests a strong link between the [CII]-deficit and the local physical conditions instead of global properties.

astro-ph.GA↗

arXiv2025

Stability analysis of the flow in a coflowing device

We analyze the stability of the coflow configuration. The experiments and the global stability analysis show that the emitted jet always destabilizes before the tapering conical meniscus. This implies that the parameter conditions at which polydisperse dripping arises cannot be determined from the linear stability analysis of the steady jetting mode. Transient simulations show that the linear superposition of decaying eigenmodes triggered by an initial perturbation can lead to the jet breakup. The breakup process significantly depends on the initial perturbation. These results question the validity of the linear stability analysis as applied to the coflowing and other similar configurations.

physics.flu-dyn↗

arXiv2025

Dense clumps survive in the vicinity of R136 in 30 Doradus

Context: The young massive cluster R136 at the center of 30 Doradus (30 Dor) in the Large Magellanic Cloud (LMC) generates a cavity in the surrounding molecular cloud. However, there is molecular gas between 2 and 10 pc in projection from R136's center. The region, known as the Stapler nebula, hosts the closest known molecular gas clouds to R136. Aims: We investigated the properties of molecular gas in the Stapler nebula to better understand why these clouds survive so close in projection to R136. Methods: We used Atacama Large Millimeter/Sub-millimeter Array 7m observations in Band 7 (345 GHz) of continuum emission, $^{12}$CO and $^{13}$CO, together with dense gas tracers CS, HCO$^+$, and HCN. Our observations resolve the molecular clouds in the nebula into individual, parsec-sized clumps. We determined the physical properties of the clumps using both dust and molecular emission, and compared the emission properties observed close to R136 to other clouds in the LMC. Results: The densest clumps in our sample, where we observe CS, HCO$^+$, and HCN, are concentrated in a northwest-southeast diagonal seen as a dark dust lane in HST images. Resolved clumps have masses between $\sim 200-2500$ \Msun, and the values obtained using the virial theorem are larger than the masses obtained through $^{12}$CO and $^{12}$CO luminosity. The velocity dispersion of the clumps is due both to self-gravity and the external pressure of the gas. Clumps at the center of our map, which have detections of dense gas tracers ($n_{crit}\sim10^6$ cm$^{-3}$ and above), are spatially coincident with young stellar objects. Conclusions: The clumps' physical and chemical properties are consistent with other clumps in 30 Dor. We suggest that these clumps are the densest regions of a Molecular Cloud carved by the radiation of R136.

astro-ph.GA↗

arXiv2023

SuperCAM CO(3-2) APEX survey at 6 pc resolution in the Small Magellanic Clouds

We present the CO(3-2) APEX survey at 6 pc resolution of the bar of the SMC. We aboard the CO analysis in the SMC-Bar comparing the CO(3-2) survey with that of the CO(2-1) of similar resolution. We study the CO(3-2)-to-CO(2-1) ratio (R32) that is very sensitive to the environment properties (e.g., star-forming regions). We analyzed the correlation of this ratio with observational quantities that trace the star formation as the local CO emission, the Spitzer color [70/160], and the total IR surface brightness measured from the Spitzer and Herschel bands. For the identification of the CO(3-2) clouds, we used the CPROPS algorithm, which allowed us to measure the physical properties of the clouds. We analyzed the scaling relationships of such physical properties. We obtained an R32 of 0.65 as a median value for the SMC, with a standard deviation of 0.3. We found that R32 varies from region to region, depending on the star formation activity. In regions dominated by HII and photo-dissociated regions (e.g., N22, N66), R32 tends to be higher than the median values. Meanwhile, lower values were found toward quiescent clouds. We also found that R32 correlates positively with the IR color [70/160] and the total IR surface brightness. This finding indicates that R32 increases with environmental properties like the dust temperature, the total gas density, and the radiation field. We have identified 225 molecular clouds with sizes R > 1.5 pc and signal-to-noise (S/N) ratio > 3 and only 17 well-resolved CO(3-2) clouds increasing the S/N ratio to > 5. These 17 clouds follow consistent scaling relationships to the inner Milky Way clouds but with some departure. The CO(3-2) tends to be less turbulent and less luminous than the inner Milky Way clouds of similar size. Finally, we estimated a median virial-based CO-to-H2 conversion factor of 12.6_{-7}^{+10} Msun/(K km s^{-1} pc^{2}) for the total sample.

astro-ph.GA↗

arXiv2022

The role of charge relaxation in electrified tip streaming

We study experimentally and numerically the onset of tip streaming in an electrified droplet. The experiments show that, for a sufficiently small dimensionless conductivity, the droplet apex oscillates before ejecting a liquid jet. This effect is caused by the limited charge transfer from the bulk to the interface. This reduces the electrostatic pressure at the droplet's stretching tip, preventing liquid ejection. This reduction of electrostatic pressure is compensated for by the electric shear stress arising during the apex oscillations, which eventually leads to the jet formation. The stability limit calculated from the global stability analysis perfectly agrees with the experimental results. However, this analysis predicts non-oscillatory, non-localized instability in all the cases, suggesting that both the oscillatory behavior and the small local scale characterizing tip streaming arise during the nonlinear droplet deformation.

physics.flu-dyn↗

arXiv2022

CO(2-1) Survey at 9 parsec resolution in the SMC

The Small Magellanic Cloud (SMC) is the closest low-metallicity galaxy to the Milky Way where the dynamical state of molecular clouds can be analyzed. We present a CO(2-1) survey at 9 pc resolution obtained with the APEX telescope in an extensive region of the SMC and characterize the properties of the molecular clouds. We study the dynamical state and stability of these clouds uniformly. We identify 177 molecular clouds within the SMC by using CPROPS, of which 124 clouds are fully resolved with signal-to-noise ratio $>$ 5. The scaling relationships show that the SMC clouds are (on average) less turbulent and less luminous than their inner Milky Way counterparts of similar size by a factor of 2 and 3, respectively, while for a fixed linewidth, the SMC clouds are over-luminous by a factor of 3.5. Using the virial masses, we derive a CO-to-H2 conversion factor for the SMC CO clouds of 10.5 M$_{\odot}(K km s^{-1} pc^{2})^{-1}$, measured at 9 pc resolution. We also determine a dust-based conversion factor of 28 M$_{\odot}(K km s^{-1} pc^{2})^{-1}$, obtained at 12 pc resolution. We find that the SMC clouds appear to be in approximate gravitational virial equilibrium. We find that the cumulative mass functions based on both the luminous mass and the virial mass are steeper than $\frac{dN}{dM} \propto M^{-2}$, suggesting that most of the molecular mass of the SMC is contained in low-mass clouds.

astro-ph.GA↗

arXiv2021

The EDGE-CALIFA survey: The resolved star formation efficiency and local physical conditions

We measure the star formation rate (SFR) per unit gas mass and the star formation efficiency (SFE$_{\rm gas}$ for total gas, SFE$_{\rm mol}$ for the molecular gas) in 81 nearby galaxies selected from the EDGE-CALIFA survey, using $^{12}$CO(J=1-0) and optical IFU data. For this analysis we stack CO spectra coherently by using the velocities of H$\alpha$ detections to detect fainter CO emission out to galactocentric radii $r_{\rm gal} \sim 1.2 r_{25}$ ($\sim 3 R_{\rm e}$), and include the effects of metallicity and high surface densities in the CO-to-H$_2$ conversion. We determine the scale lengths for the molecular and stellar components, finding a close to 1:1 relation between them. This result indicates that CO emission and star formation activity are closely related. We examine the radial dependence of SFE$_{\rm gas}$ on physical parameters such as galactocentric radius, stellar surface density $\Sigma_{\star}$, dynamical equilibrium pressure $P_{\rm DE}$, orbital timescale $\tau_{\rm orb}$, and the Toomre $Q$ stability parameter (including star and gas $Q_{\rm star+gas}$). We observe a generally smooth, continuous exponential decline in the SFE$_{\rm gas}$ with $r_{\rm gal}$. The SFE$_{\rm gas}$ dependence on most of the physical quantities appears to be well described by a power-law. Our results also show a flattening in the SFE$_{\rm gas}$-$\tau_{\rm orb}$ relation at $\log[\tau_{\rm orb}]\sim 7.9-8.1$ and a morphological dependence of the SFE$_{\rm gas}$ per orbital time, which may reflect star formation quenching due to the presence of a bulge component. We do not find a clear correlation between SFE$_{\rm gas}$ and $Q_{\rm star+gas}$.

astro-ph.GA↗

arXiv2021

Molecular gas properties of Q1700-MD94: a massive, main-sequence galaxy at $z\approx2$

We use a combination of new NOrthern Extended Millimeter Array (NOEMA) observations of the pair of [CI] transitions, the CO(7-6) line, and the dust continuum, in addition to ancillary CO(1-0) and CO(3-2) data, to study the molecular gas properties of Q1700-MD94, a massive, main-sequence galaxy at $z\approx2$. We find that for a reasonable set of assumptions for a typical massive star-forming galaxy, the CO(1-0), the [CI](1-0) and the dust continuum yield molecular gas masses that are consistent within a factor of $\sim2$. The global excitation properties of the molecular gas as traced by the [CI] and CO transitions are similar to those observed in other massive, star-forming galaxies at $z\sim2$. Our large velocity gradient (LVG) modeling using RADEX of the CO and [CI] spectral line energy distributions (SLEDs) suggests the presence of relatively warm ($T_{\rm kin}=41$K), dense ($n_{\rm H_2}=8\times10^{3}~{\rm cm}^{-3}$) molecular gas, comparable to the high-excitation molecular gas component observed in main-sequence, star-forming galaxies at $z\sim1$. The galaxy size in the CO(1-0) and CO(7-6) line emission are comparable, which suggests that the highly-excited molecular gas is distributed throughout the disk powered by intense star formation activity. To confirm this scenario will require spatially resolved observations of the CO and [CI] lines which can now be obtained with NOEMA upgraded capabilities.

astro-ph.GA↗

arXiv2021

Effect of an axial electric field on the breakup of a leaky-dielectric liquid filament

We study experimentally and numerically the thinning of a Newtonian leaky-dielectric filament subject to an axial electric field. We consider moderately viscous liquids with high permittivity. The experiments show that satellite droplets are produced from the breakup of the filaments with high electrical permittivity due to the action of the electric field. Two electrified filaments with the same minimum radius thin at the same speed regardless of when the voltage was applied. The numerical simulations show that the polarization stress is responsible for the pinching delay observed in the experiments. Asymptotically close to the pinching point, the filament pinching is dominated by the diverging hydrodynamic forces. The polarization stress becomes subdominant even if this stress also diverges at this finite-time singularity.

physics.flu-dyn↗

arXiv2020

Resolved star formation in the metal poor star-forming region Magellanic Bridge C

Magellanic Bridge C (MB-C) is a metal-poor ($\sim$1/5 $Z_{\odot}$) low-density star-forming region located 59 kpc away in the Magellanic Bridge, offering a resolved view of the star formation process in conditions different to the Galaxy. From Atacama Large Millimetre Array CO (1-0) observations, we detect molecular clumps associated to candidate young stellar objects (YSOs), pre-main sequence (PMS) stars, and filamentary structure identified in far-infrared imaging. YSOs and PMS stars form in molecular gas having densities between 17-200 $M_{\odot}$ pc$^{-2}$, and have ages between $\lesssim$0.1-3 Myr. YSO candidates in MB-C have lower extinction than their Galactic counterparts. Otherwise, our results suggest that the properties and morphologies of molecular clumps, YSOs, and PMS stars in MB-C present no patent differences with respect to their Galactic counterparts, tentatively alluding that the bottleneck to forming stars in regions similar to MB-C is the conversion of atomic gas to molecular.

astro-ph.GA↗

arXiv2020

Global stability analysis of axisymmetric liquid-liquid flow focusing

We analyze both numerically and experimentally the stability of the steady jetting tip streaming produced by focusing a liquid stream with another liquid current when they coflow through the orifice of an axisymmetric nozzle. We calculate the global eigenmodes characterizing the response of this configuration to small-amplitude perturbations. In this way, the critical conditions leading to the instability of the steady jetting tip streaming are determined. The unstable perturbations are classified according to their oscillatory character and to the region where they are originated (convective and absolute instability). We derive and explain in terms of the velocity field a simple scaling law to predict the diameter of the emitted jet. The numerical stability limits are compared with experimental results finding reasonable agreement. The experiments confirm the existence of the two instability mechanisms predicted by the global stability analysis.

physics.flu-dyn↗

arXiv2020

Studying star-forming processes at core and clump scales: the case of the young stellar object G29.862-0.0044

Massive molecular clumps fragment into cores where star formation takes place, hence star-forming studies should be done at different spatial scales. Using near-IR data obtained with Gemini, data of CH3OCHO and CH3CN from the ALMA database, observations of HCN, HNC, HCO+, and C2H carried out with ASTE, and CO data from public surveys, we perform a deep study of the YSO G29.86-0.004 at core and clump spatial scales. The near-IR emission shows two nebulosities separated by a dark lane, suggesting a typical disk-jets system, but highly asymmetric. They are likely produced by scattered light in cavities carved out by jets on an infalling envelope of material, which also present line emission of H2 and [FeII]. The presence of the complex molecular species observed with ALMA confirms that we are mapping a hot molecular core. The CH3CN emission concentrates at the position of the dark lane and it appears slightly elongated from southwest to northeast in agreement with the inclination of the system as observed at near-IR. The morphology of the CH3OCHO emission is more complex and extends along some filaments and concentrates in knots and clumps, mainly southwards the dark-lane, suggesting that the southern jet is encountering a dense region. The northern jet flows more freely, generating more extended features. This is in agreement with the red-shifted molecular outflow traced by the 12CO J=3-2 line extending towards the northwest and the lack of a blue-shifted outflow. This configuration can be explained by considering that the YSO is located at the furthest edge of the molecular clump along the line of sight, which is consistent with the position of the source in the cloud mapped in the C18O J=3-2 line. The detection of HCN, HNC, HCO+, and C2H allowed us to characterize the dense gas at clump scales, yielding results that are in agreement with the presence of a high-mass protostellar object.

astro-ph.GA↗

arXiv2020

ALMA resolves molecular clouds in the metal poor Magellanic Bridge A

(Abridged)We characterize gas and dust emission in Magellanic Bridge A, which has the highest 870$\mu$m excess of emission found in single dish surveys. Using the ALMA telescope, we mapped the Magellanic Bridge A molecular cloud with sub-parsec resolution, in 1.3 mm continuum and CO(2-1) line emission. We also map the cloud in 870$\mu$m continuum and CO(2-1) line emission at ~6 pc resolution with APEX. We combine the ALMA and APEX CO(2-1) line cubes to study the molecular gas emission. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and CO(2-1) emission, which we call North and South. Dust emission in the North source, according to our best parameters from fitting the far-infrarred fluxes, is ~3 K colder than in the South source in correspondence to its less developed star formation. Both dust sources present large submillimeter excesses in LABOCA data: according to our best fits the excess over the modified blackbody (MBB) fit to the Spitzer/Herschel continuum are ~7 and ~3 for the North and South sources respectively. Nonetheless, we do not detect the corresponding 1.3 mm continuum with ALMA. Our limits are compatible with the extrapolation of the MBB fits and therefore we cannot independently confirm the excess at this longer wavelength. The CO(2-1) emission is in two parsec-sized clouds with virial masses around 400 and 700 Mo each. Their volume densities are ~700-2600 cm$^{-3}$, larger than typical bulk densities of Galactic molecular clouds. The CO-to-H2 conversion factor is 6.5 and 15 M$_{\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ for the North and South clouds, calculated using their respective virial masses and CO(2-1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses $M\sim1.3\times10^3$ M$_{\odot}$ and $2.9\times10^3$ M$_{\odot}$ for North and South respectively, a factor ~4 larger than the virial masses we infer from CO.

astro-ph.GA↗

arXiv2020

Influence of the surface viscous stress on the pinch-off of free surfaces loaded with nearly-inviscid surfactants

We analyze both theoretically and experimentally the breakup of a pendant water droplet loaded with Sodium Dodecyl Sulfate (SDS). The free surface minimum radius measured in the experiments is compared with that obtained from a numerical solution of the full Navier-Stokes equations for different values of the shear and dilatational surface viscosities. This comparison shows the small but measurable effect of the surface viscous stresses on the system dynamics for sufficiently small spatiotemporal distances from the breakup point, and allows to establish upper bounds for the values of the shear and dilatational viscosities. We study numerically the distribution of Marangoni and viscous stresses over the free surface as a function of the time to the pinching, and describe how surface viscous stresses grow in the pinching region as the free surface approaches its breakup. When Marangoni and surface viscosity stresses are taken into account, the surfactant is not swept away from the thread neck in the time interval analyzed. Surface viscous stresses eventually balance the driving capillary pressure in that region for small enough values of the time to pinching. Based on this result, we propose a scaling law to account for the effect of the surface viscosities on the last stage of the temporal evolution of the neck radius.

physics.flu-dyn↗

arXiv2019

Kinematic study of the molecular gas associated with two cometary globules in Sh2-236

Cometary globules, dense molecular gas structures exposed to the UV radiation, are found inside HII regions. Understanding the nature and origin of these structures through a kinematic study of the molecular gas is useful to advance in our knowledge of the interplay between radiation and molecular gas. Using ASTE we carried out molecular observations towards two cometary globules (Sim129 and Sim130) in the HII region Sh2-236. We mapped two regions with the 12CO J=3-2 and HCO+ J=4-3 lines. Additionally, two single pointings of C2H (N=4-3), HNC and HCN J=4-3 were observed. We combined our observations with public infrared and optical data to analyse the distribution and kinematics of the gas. We found kinematic signatures of infalling gas in the 12CO J=3-2 and C2H J=4-3 spectra towards Sim129 . We detected HCO+, HCN, and HNC J=4-3 only towards Sim130. The HCN/HNC integrated ratio of about 3 found in Sim130 suggests that the possible star formation activity within the globule has not yet ionized the gas. The location of NVSS 052255+33315, which peaks towards the brightest border of the globule, supports this scenario. The non-detection of these molecules towards Sim129 could be due to the radiation arising from the star formation activity inside this globule. The ubiquitous presence of the C2H molecule towards both globules shows the action of nearby O-B stars irradiating their external layers. Based on mid-infrared emission, we identified two new structures: a region of diffuse emission (R1) located, in projection, in front of the head of Sim129, and a pillar-like feature (P1) placed besides Sim130. Based on 12CO J=3-2, we found molecular gas associated with Sim129, Sim130, R1 and P1 at radial velocities of -1.5, -11, +10, and +4 km/s, respectively. Therefore, while Sim129 and P1 are located at the far side of the shell, Sim130 is placed at the near side, consistent with earlier results.

astro-ph.GA↗

arXiv2019

Molecular gas in the outflow of the Small Magellanic Cloud

We report the first evidence of molecular gas in two atomic hydrogen (HI) clouds associated with gas outflowing from the Small Magellanic Cloud (SMC). We used the Atacama Pathfinder Experiment (APEX) to detect and spatially resolve individual clumps of CO(2-1) emission in both clouds. CO clumps are compact (~ 10 pc) and dynamically cold (linewidths < 1 km/s). Most CO emission appears to be offset from the peaks of the HI emission, some molecular gas lies in regions without a clear HI counterpart. We estimate a total molecular gas mass of 10^3-10^4 Msun in each cloud and molecular gas fractions up to 30% of the total cold gas mass (molecular + neutral). Under the assumption that this gas is escaping the galaxy, we calculated a cold gas outflow rate of 0.3-1.8 Msun/yr and mass loading factors of 3 -12 at a distance larger than 1 kpc. These results show that relatively weak star-formation-driven winds in dwarf galaxies like the SMC are able to accelerate significant amounts of cold and dense matter and inject it into the surrounding environment.

astro-ph.GA↗

arXiv2019

Millimeter and far-IR study of the IRCD SDC 341.232-0.268

We analyze the molecular gas and dust associated with the infrared dark cloud SDC 341.232-0.268 in order to investigate the characteristics and parameters of the gas, determine the evolutionary status of four embedded EGO candidates and establish possible infall or outflow gas motions. We based our study on $^{12}$CO(2-1), $^{13}$CO(2-1), and C$^{18}$O(2-1) data obtained with the APEX telescope, molecular data of high density tracers from the MALT90 survey and IR images from Spitzer, Herschel and ATLASGAL. The study reveals two clumps at -44 km/s towards the IRDC, with densities of > 10$^4$ cm$^{-3}$ , typical of IRDCs, while high density tracers show H$_2$ densities > 10$^5$ . FIR images reveals the presence of cold dust linked to the molecular clumps and EGOs. A comparison of spectra of the optically thin and optically thick molecular lines towards the EGOs suggests the existence of infall and outflow motions.

astro-ph.GA↗