arXiv subjects

Manuel Fernández-López

Publications and source records attributed to Manuel Fernández-López.

31 records · Page 2Linked to original sources

arXiv2021

ALMA CN Zeeman Observations of AS 209: Limits on Magnetic Field Strength and Magnetically Driven Accretion Rate

While magnetic fields likely play an important role in driving the evolution of protoplanetary disks through angular momentum transport, observational evidence of magnetic fields has only been found in a small number of disks. Although dust continuum linear polarization has been detected in an increasing number of disks, its pattern is more consistent with that from dust scattering than from magnetically aligned grains in the vast majority of cases. Continuum linear polarization from dust grains aligned to a magnetic field can reveal information about the magnetic field's direction, but not its strength. On the other hand, observations of circular polarization in molecular lines produced by Zeeman splitting offer a direct measure of the line-of-sight magnetic field strength in disks. We present upper limits on the net toroidal and vertical magnetic field strengths in the protoplanetary disk AS 209 derived from Zeeman splitting observations of the CN 2-1 line. The 3$σ$ upper limit on the net line-of-sight magnetic field strength in AS 209 is 5.0 mG on the redshifted side of the disk and 4.2 mG on the blueshifted side of the disk. Given the disk's inclination angle, we set a 3$σ$ upper limit on the net toroidal magnetic field strength of 8.7 and 7.3 mG for the red and blue sides of the disk, respectively, and 6.2 and 5.2 mG on the net vertical magnetic field on the red and blue sides of the disk. If magnetic disk winds are a significant mechanism of angular momentum transport in the disk, magnetic fields of a strength close to the upper limits would be sufficient to drive accretion at the rate previously inferred for regions near the protostar.

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arXiv2020

Confirming the Explosive Outflow in G5.89 with ALMA

The explosive molecular outflow detected decades ago in the Orion BN/KL region of massive star formation was considered to be a bizarre event. This belief was strengthened by the non detection of similar cases over the years with the only exception of the marginal case of DR21. Here, we confim a similar explosive outflow associated with the UCH$_{\rm II}$ region G5.89$-$0.39 that indicates that this phenomenon is not unique to Orion or DR21. Sensitive and high angular resolution ($\sim$ 0.1$''$) ALMA CO(2$-$1) and SiO(5$-$4) observations show that the molecular outflow in the massive star forming region G5.89$-$0.39 is indeed an explosive outflow with an age of about 1000 yrs and a liberated kinetic energy of 10$^{46-49}$ erg. Our new CO(2$-$1) ALMA observations revealed over 30 molecular filaments, with Hubble-like expansion motions, pointing to the center of UCH$_{\rm II}$ region. In addition, the SiO(5$-$4) observations reveal warmer and strong shocks very close to the origin of the explosion, confirming the true nature of the flow. A simple estimation for the occurrence of these explosive events during the formation of the massive stars indicates an event rate of once every $\sim$100 yrs, which is close to the supernovae rate.

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arXiv2020

Four annular structures in a protostellar disk less than 500,000 years old

Annular structures, or rings and gaps, in disks around pre-main sequence stars have been detected in abundance towards Class II objects ~1,000,000 years in age. These structures are often interpreted as evidence of planet formation, with planet-mass bodies carving rings and gaps in the disk. This implies that planet formation may already be underway in even younger disks in the Class I phase, when the protostar is still embedded in a larger scale dense envelope of gas and dust. While younger disks likely play an important role in the onset of planet formation, only within the past decade have detailed properties of disks in the youngest star-forming phases begun to be observed. Here we present 1.3 mm dust emission observations with 5 au resolution that show four annular substructures in the disk of the young (<500,000 years) protostar IRS 63. IRS 63, a single Class I source located in the nearby Ophiuchus molecular cloud (at a distance of 144 pc), is one of the brightest Class I protostars at (sub)millimeter wavelengths that also has a relatively large disk (>50 au). Multiple annular substructures observed towards disks at young times can act as an early foothold for dust grain growth, a prerequisite of planet formation. Whether planets already exist or not in the disk of IRS 63, it is clear that the planet formation process begins in the young protostellar phases, earlier than predicted by current planet formation theories.

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arXiv2020

Tidal Interaction between the UX Tauri Disk A/C System Revealed by ALMA

We present sensitive and high angular resolution ($\sim$0.2-0.3$''$) (sub)millimeter (230 and 345 GHz) continuum and CO(2$-$1)/CO(3$-$2) line archive observations of the disk star system in UX Tauri carried out with ALMA (The Atacama Large Millimeter/Submillimeter Array). These observations reveal the gas and dusty disk surrounding the young star UX Tauri A with a large signal-to-noise ratio ($>$400 in the continuum and $>$50 in the line), and for the first time is detected the molecular gas emission associated with the disk of UX Tauri C (with a size for the disk of $<$56 au). No (sub)millimeter continuum emission is detected at 5$σ$-level (0.2 mJy at 0.85 mm) associated with UX Tauri C. For the component UX Tauri C, we estimate a dust disk mass of $\leq$ 0.05 M$_\oplus$. Additionally, we report a strong tidal disk interaction between both disks UX Tauri A/C, separated 360 au in projected distance. The CO line observations reveal marked spiral arms in the disk of UX Tauri A and an extended redshifted stream of gas associated with the UX Tauri C disk. No spiral arms are observed in the dust continuum emission of UX Tauri A. Assuming a Keplerian rotation we estimate the enclosed masses (disk$+$star) from their radial velocities in 1.4 $\pm$ 0.6 M$_\odot$ for UX Tauri A, and 70 $\pm$ 30 / $\sin i$ Jupiter masses for UX Tauri C (the latter coincides with the mass upper limit value for a brown dwarf). The observational evidence presented here lead us to propose that UX Tauri C is having a close approach of a possible wide, evolving and eccentric orbit around the disk of UX Tauri A causing the formation of spiral arms and the stream of molecular gas falling towards UX Tauri C.

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arXiv2019

Polarization in Disks

Polarized dust emission outside of disks reveal the magnetic field morphology of molecular clouds. Within disks, however, polarized dust emission can arise from very different mechanisms (e.g., self-scattering), and each of them are useful for constraining physical properties in the disk. For example, these mechanisms allow us to constrain the disk grain size distributions and grain/disk geometries, independent from current methods of measuring these parameters. To accurately model these features and disentangle the various polarization mechanisms, multiwavelength observations at very high resolution and sensitivity are required. With significant upgrades to current interferometric facilities, we can understand how grains evolve in disks during the planet formation process.

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arXiv2018

ALMA Observations of Polarized 872 $μ$m Dust Emission from the Protostellar Systems VLA 1623 and L1527

We present high-sensitivity ($σ_I\sim0.2-0.5$ mJy, $σ_{QU}\sim0.05$ mJy), high-resolution (0.12-0.2 arcsec) observations of polarized 872 $μ$m dust emission from the young multiple system VLA 1623 in $ρ$ Ophiuchus and the protostar L1527 in Taurus. We detect the circumstellar material of VLA 1623A, the extended Keplerian disk surrounding VLA 1623A which we call VLA 1623CBdisk, VLA 1623B, VLA 1623W, and L1527 strongly in the polarized emission, at the $\sim1-3\%$ level. We spatially resolve VLA 1623A into two sources, VLA 1623Aa and VLA 1623Ab, separated by $\sim30$ au and located within a cavity of radius $\sim50$ au within the circumbinary Keplerian disk, as well as the edge-on disk of VLA 1623W. The polarization angle of the emission is uniform across each protostellar source and nearly coincides with each disk's minor axis. The offsets between the minor axis and the polarization angle are not uniformly distributed at the $P\lesssim2\times10^{-4}$ level. The circumbinary disk surrounding VLA 1623Aab is azimuthally symmetrically polarized. Each compact source's emission is partially optically thick ($τ\gtrsim1$) at 872 $μ$m, complicating interpretations of polarization involving aligned grains. We find evidence against alignment by radiative flux in each source, particularly in the edge-on VLA 1623W and L1527. We detect astrometric offsets between the polarized emission and the total intensity in VLA 1623Aa, VLA 1623Ab, and VLA 1623B, as predicted if self-scattering in the optically thick limit operates. We conclude that self-scattering is likely responsible for disk-scale polarization at 872 $μ$m in these systems.

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arXiv2018

Distribution of Serpens South protostars revealed with ALMA

Aims: We investigated the masses and spatial distributions of pre-stellar and protostellar candidates in the young, low-mass star forming region Serpens South, where active star formation is known to occur along a predominant filamentary structure. Previous observations used to study these distributions have been limited by two important observational factors: (1) sensitivity limits that leave the lowest-mass sources undetected, or (2) resolution limits that cannot distinguish binaries and/or cluster members in close proximity. Methods: Recent millimeter-wavelength interferometry observations can now uncover faint and/or compact sources in order to study a more complete population of protostars, especially in nearby ($D<500$ pc) clusters. Here we present ALMA observations of 1 mm (Band 6) continuum in a $3 \times 2$ arcminutes region at the center of Serpens South. Our angular resolution of $\sim1$ arcsec is equivalent to $\sim400$ au, corresponding to scales of envelopes and/or disks of protostellar sources. Results: We detect 52 sources with 1 mm continuum, and we measure masses of $0.002 - 0.9$ solar masses corresponding to gas and dust in the disk and/or envelope of the protostellar system. For the deeply embedded (youngest) sources with no IR counterparts, we find evidence of mass segregation and clustering according to: the Minimum Spanning Tree method, distribution of projected separations between unique sources, and concentration of higher-mass sources near to the dense gas at the cluster center. Conclusions: The mass segregation of the mm sources is likely primordial rather than dynamical given the young age of this cluster, compared with segregation time. This is the first case to show this for mm sources in a low-mass protostellar cluster environment.

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arXiv2018

ALMA Reveals A Collision Between Protostellar Outflows in BHR71

For a binary protostellar outflow system in which its members are located so close to each other (the separation being smaller than the addition of the widths of the flows) and with large opening angles, the collision seems unavoidable regardless of the orientation of the outflows. This is in contrast to the current observational evidence of just a few regions with indications of colliding outflows. Here, using sensitive observations of the Atacama Large Millimeter/Submillimeter Array (ALMA), we report resolved images of carbon monoxide (CO) towards the binary flows associated with the BHR71 protostellar system. These images reveal for the first time solid evidence that their flows are partially colliding, increasing the brightness of the CO, the dispersion of the velocities in the interaction zone, and changing part of the orientation in one of the flows. Additionally, this impact opened the possibility of knowing the 3D geometry of the system, revealing that one of its components (IRS2) should be closer to us.

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arXiv2017

ALMA Reveals Transition of Polarization Pattern with Wavelength in HL Tau's Disk

The mechanism for producing polarized emission from protostellar disks at (sub)millimeter wavelengths is currently uncertain. Classically, polarization is expected from non-spherical grains aligned with the magnetic field. Recently, two alternatives have been suggested. One polarization mechanism is caused by self-scattering from dust grains of sizes comparable to the wavelength while the other mechanism is due to grains aligned with their short axes along the direction of radiation anisotropy. The latter has recently been shown as a likely mechanism for causing the dust polarization detected in HL Tau at 3.1 mm. In this paper, we present ALMA polarization observations of HL Tau for two more wavelengths: 870 $μ$m and 1.3 mm. The morphology at 870 $μ$m matches the expectation for self-scattering, while that at 1.3 mm shows a mix between self-scattering and grains aligned with the radiation anisotropy. The observations cast doubt on the ability of (sub)millimeter continuum polarization to probe disk magnetic fields for at least HL Tau. By showing two distinct polarization morphologies at 870 $μ$m and 3.1 mm and a transition between the two at 1.3 mm, this paper provides definitive evidence that the dominant (sub)millimeter polarization mechanism transitions with wavelength. In addition, if the polarization at 870 $μ$m is due to scattering, the lack of polarization asymmetry along the minor axis of the inclined disk implies that the large grains responsible for the scattering have already settled into a geometrically thin layer, and the presence of asymmetry along the major axis indicates that the HL Tau disk is not completely axisymmetric.

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arXiv2016

CARMA Large Area Star Formation Survey: Dense Gas in the Young L1451 Region of Perseus

We present a 3 mm spectral line and continuum survey of L1451 in the Perseus Molecular Cloud. These observations are from the CARMA Large Area Star Formation Survey (CLASSy), which also imaged Barnard 1, NGC 1333, Serpens Main and Serpens South. L1451 is the survey region with the lowest level of star formation activity---it contains no confirmed protostars. HCO+, HCN, and N2H+ (J=1-0) are all detected throughout the region, with HCO+ the most spatially widespread, and molecular emission seen toward 90% of the area above N(H_2) column densities of 1.9x10^21 cm^-2. HCO+ has the broadest velocity dispersion, near 0.3 km/s on average, compared to ~0.15 km/s for the other molecules, thus representing a range from supersonic to subsonic gas motions. Our non-binary dendrogram analysis reveals that the dense gas traced by each molecule has similar hierarchical structure, and that gas surrounding the candidate first hydrostatic core (FHSC), L1451-mm, and other previously detected single-dish continuum clumps have similar hierarchical structure; this suggests that different sub-regions of L1451 are fragmenting on the pathway to forming young stars. We determined the three-dimensional morphology of the largest detectable dense gas structures to be relatively ellipsoidal compared to other CLASSy regions, which appeared more flattened at largest scales. A virial analysis shows the most centrally condensed dust structures are likely unstable against collapse. Additionally, we identify a new spherical, centrally condensed N2H+ feature that could be a new FHSC candidate. The overall results suggest L1451 is a young region starting to form its generation of stars within turbulent, hierarchical structures.

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arXiv2014

Spatially Resolved Magnetic Field Structure in the Disk of a T Tauri Star

Magnetic fields in accretion disks play a dominant role during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars themselves and possibly in the innermost part of the accretion disk, but the strength and morphology of the field in the bulk of the disk have not been observed. Unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicular to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal, or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau. Here we report resolved measurements of the polarized 1.25 mm continuum emission from HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the major axis (~210 AU diameter) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field does not fit the data well either. The unexpected morphology suggests that the magnetic field's role for the accretion of a T Tauri star is more complex than the current theoretical understanding.

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arXiv2013

Multiple monopolar outflows driven by massive protostars in IRAS 18162-2048

In this paper we present Combined Array for Research in Millimeter-wave Astronomy (CARMA) 3.5 mm observations and SubMillimeter Array (SMA) 870 microns observations toward the high-mass star-forming region IRAS 1816-2048, the core of the HH 80/81/80N system. Molecular emission from HCN, HCO+ and SiO is tracing two molecular outflows (the so-called Northeast and Northwest outflows). These outflows have their origin in a region close to the position of MM2, a millimeter source known to harbor a couple protostars. We estimate for the first time the physical characteristics of these molecular outflows, which are similar to those of 1000-5000 Lo protostars, suggesting that MM2 harbors high-mass protostars. High-angular resolution CO observations show an additional outflow due southeast. We identify for the first time its driving source, MM2(E), and see evidence of precession. All three outflows have a monopolar appearance, but we link the NW and SE lobes, explaining their asymmetric shape as a consequence of possible deflection.

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arXiv2011

A rotating molecular disk toward IRAS 18162-2048, the exciting source of HH 80-81

We present several molecular line emission arcsec and subarcsec observations obtained with the Submillimeter Array (SMA) in the direction of the massive protostar IRAS 18162-2048, the exciting source of HH 80-81. The data clearly indicates the presence of a compact (radius~425-850 AU) SO2 structure, enveloping the more compact (radius~150 AU) 1.4 millimeter dust emission (reported in a previous paper). The emission spatially coincides with the position of the prominent thermal radio jet which terminates at the HH 80-81 and HH 80N Herbig-Haro objects. Furthermore, the molecular emission is elongated in the direction perpendicular to the axis of the thermal radio jet, suggesting a disk-like structure. We derive a total dynamic mass (disk-like structure and protostar) of 11-15 msun. The SO2 spectral line data also allow us to constrain the structure temperature between 120-160 K and the volume density > 2x10^9 cm-3. We also find that such a rotating flattened system could be unstable due to gravitational disturbances. The data from C17O line emission show a dense core within this star-forming region. Additionally, the H2CO and the SO emissions appear clumpy and trace the disk-like structure, a possible interaction between a molecular core and the outflows, and in part, the cavity walls excavated by the thermal radio jet.

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