arXiv subjects

Nadia M. Murillo

Publications and source records attributed to Nadia M. Murillo.

29 records · Page 2Linked to original sources

arXiv2019

Chronology of Episodic Accretion in Protostars -- an ALMA survey of the CO and H$_2$O snowlines

Episodic accretion has been used to explain the wide range of protostellar luminosities, but its origin and influence on the star forming process are not yet fully understood. We present an ALMA survey of N$_2$H$^+$ ($1-0$) and HCO$^+$ ($3-2$) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N$_2$H$^+$ and HCO$^+$ are destroyed via gas-phase reactions with CO and H$_2$O, respectively, thus tracing the CO and H$_2$O snowline locations. A snowline location at a much larger radius than that expected from the current luminosity suggests that an accretion burst has occurred in the past which has shifted the snowline outward. We identified 18/18 Class 0 and 9/10 Class I post-burst sources from N$_2$H$^+$, and 7/17 Class 0 and 1/8 Class I post-burst sources from HCO$^+$.The accretion luminosities during the past bursts are found to be $\sim10-100~L_\odot$. This result can be interpreted as either evolution of burst frequency or disk evolution. In the former case, assuming that refreeze-out timescales are 1000 yr for \ce{H2O} and 10,000 yr for CO, we found that the intervals between bursts increases from 2400 yr in the Class 0 to 8000 yr in the Class I stage. This decrease in the burst frequency may reflect that fragmentation is more likely to occur at an earlier evolutionary stage when the young stellar object is more prone to instability.

astro-ph.GA↗

arXiv2019

Dynamic Fault Tolerance Through Resource Pooling

Miniaturized satellites are currently not considered suitable for critical, high-priority, and complex multi-phased missions, due to their low reliability. As hardware-side fault tolerance (FT) solutions designed for larger spacecraft can not be adopted aboard very small satellites due to budget, energy, and size constraints, we developed a hybrid FT-approach based upon only COTS components, commodity processor cores, library IP, and standard software. This approach facilitates fault detection, isolation, and recovery in software, and utilizes fault-coverage techniques across the embedded stack within an multiprocessor system-on-chip (MPSoC). This allows our FPGA-based proof-of-concept implementation to deliver strong fault-coverage even for missions with a long duration, but also to adapt to varying performance requirements during the mission. The operator of a spacecraft utilizing this approach can define performance profiles, which allow an on-board computer (OBC) to trade between processing capacity, fault coverage, and energy consumption using simple heuristics. The software-side FT approach developed also offers advantages if deployed aboard larger spacecraft through spare resource pooling, enabling an OBC to more efficiently handle permanent faults. This FT approach in part mimics a critical biological systems's way of tolerating and adjusting to failures, enabling graceful ageing of an MPSoC.

cs.DC↗

arXiv2018

Role of environment and gas temperature in the formation of multiple protostellar systems: molecular tracers

Context: Simulations suggest that gas heating due to radiative feedback is a key factor in whether or not multiple protostellar systems will form. Chemistry is a good tracer of the physical structure of a protostellar system, since it depends on the temperature structure. Aims: We aim to study the relationship between envelope gas temperature and protostellar multiplicity. Methods: Single dish observations of various molecules that trace the cold, warm and UV-irradiated gas are used to probe the temperature structure of multiple and single protostellar systems on 7000 AU scales. Results: Single, close binary, and wide multiples present similar current envelope gas temperatures, as estimated from H$_2$CO and DCO$^+$ line ratios. The temperature of the outflow cavity, traced by c-C$_3$H$_2$, on the other hand, shows a relation with bolometric luminosity and an anti-correlation with envelope mass. Although the envelope gas temperatures are similar for all objects surveyed, wide multiples tend to exhibit a more massive reservoir of cold gas compared to close binary and single protostars. Conclusions: Although the sample of protostellar systems is small, the results suggest that gas temperature may not have a strong impact on fragmentation. We propose that mass, and density, may instead be key factors in fragmentation.

astro-ph.SR↗

arXiv2018

Revised SED of the triple protostellar system VLA 1623-2417

VLA 1623$-$2417 is a triple protostellar system deeply embedded in Ophiuchus A. Sources A and B have a separation of 1.1", making their study difficult beyond the submillimeter regime. Lack of circumstellar gas emission suggested that VLA 1623$-$2417 B has a very cold envelope and is much younger than source A, generally considered the prototypical Class 0 source. We explore the consequences of new ALMA Band 9 data on the spectral energy distribution (SED) of VLA 1623$-$2417 and their inferred nature. Using dust continuum observations spanning from centimeter to near-infrared wavelengths, the SED of each component in VLA 1623$-$2417 is constructed and analysed. The ALMA Band 9 data presented here show that the SED of VLA 1623$-$2417 B does not peak at 850 $μ$m as previously expected, but instead presents the same shape as VLA 1623$-$2417 A at wavelengths shorter than 450 $μ$m. The results presented here indicate that the previous assumption that the flux in $Herschel$ and Spitzer observations is solely dominated by VLA 1623$-$2417 A is not valid, and instead, VLA 1623$-$2417 B most likely contributes a significant fraction of the flux at $λ~<$ 450 $μ$m. These results, however, do not explain the lack of circumstellar gas emission and puzzling nature of VLA 1623$-$2417 B.

astro-ph.SR↗

arXiv2018

Tracing the cold and warm physico-chemical structure of deeply embedded protostars: IRAS 16293-2422 versus VLA 1623-2417

Much attention has been placed on the dust distribution in protostellar envelopes, but there are still many unanswered questions regarding the structure of the gas. We aim to start identifying the factors that determine the chemical structure of protostellar regions, by studying and comparing low-mass embedded systems in key molecular tracers. The cold and warm chemical structures of two embedded Class 0 systems, IRAS16293 and VLA1623 are characterized through interferometric observations. DCO+, N2H+ and N2D+ are used to trace the spatial distribution and physics of the cold regions of the envelope, while c-C3H2 and C2H from models of the chemistry are expected to trace the warm (UV-irradiated) regions. Both sources show a number of striking similarities and differences. DCO+ consistently traces the cold material at the disk-envelope interface, where gas and dust temperatures are lowered due to disk shadowing. N2H+ and N2D+, also tracing cold gas, show low abundances towards VLA1623, but for IRAS16293, the distribution of N2D+ is consistent with the same chemical models that reproduce DCO+. c-C3H2 and C2H show different spatial distributions for the two systems. For IRAS16293, c-C3H2 traces the outflow cavity wall, while C2H is found in the envelope material but not the outflow cavity wall. In contrast, toward VLA1623 both molecules trace the outflow cavity wall. Finally, hot core molecules are abundantly observed toward IRAS16293 but not toward VLA1623. We identify temperature as one of the key factors in determining the chemical structure of protostars as seen in gaseous molecules. More luminous protostars, such as IRAS16293, will have chemical complexity out to larger distances than colder protostars, such as VLA1623. Additionally, disks in the embedded phase have a crucial role in controlling both the gas and dust temperature of the envelope, and consequently the chemical structure.

astro-ph.SR↗

arXiv2018

Probing Episodic Accretion in Very Low Luminosity Objects

Episodic accretion has been proposed as a solution to the long-standing luminosity problem in star formation; however, the process remains poorly understood. We present observations of line emission from N2H+ and CO isotopologues using the Atacama Large Millimeter/submillimeter Array (ALMA) in the envelopes of eight Very Low Luminosity Objects (VeLLOs). In five of the sources the spatial distribution of emission from N2H+ and CO isotopologues shows a clear anti-correlation. It is proposed that this is tracing the CO snow line in the envelopes: N2H+ emission is depleted toward the center of these sources in contrast to the CO isotopologue emission which exhibits a peak. The positions of the CO snow lines traced by the N2H+ emission are located at much larger radii than those calculated using the current luminosities of the central sources. This implies that these five sources have experienced a recent accretion burst because the CO snow line would have been pushed outwards during the burst due to the increased luminosity of the central star. The N2H+ and CO isotopologue emission from DCE161, one of the other three sources, is most likely tracing a transition disk at a later evolutionary stage. Excluding DCE161, five out of seven sources (i.e., ~70%) show signatures of a recent accretion burst. This fraction is larger than that of the Class 0/I sources studied by Jørgensen et al. (2015) and Frimann et al. (2016) suggesting that the interval between accretion episodes in VeLLOs is shorter than that in Class 0/I sources.

astro-ph.GA↗

arXiv2017

The cold and warm physico-chemical structure of embedded protostars

The physical evolution of low-mass protostars is relatively well-established, however, there are many open questions on the chemical structure of protostars. The chemical fingerprint generated in the early embedded phase of star formation may be transmitted to the later stages of star, planet and comet formation. The factors that influence the chemical fingerprint are then of interest to study, and determine whether the chemical structure is inherited from the parent cloud or product of the physical processes during star formation. Results of observations and modelling of molecules that trace the cold and warm extended structures of embedded protostars are briefly presented here. Two multiple protostellar systems are studied, IRAS 16293-2422 and VLA 1623-2417, both located in $ρ$ Ophiuchus. We find that the physical structure of the protostars, that is the disk(-like) strucutres, outflow cavity and different luminosities, are important factors in determining the chemical structure of these embedded protostars.

astro-ph.SR↗

arXiv2016

Do siblings always form and evolve simultaneously? Testing the coevality of multiple protostellar systems through SEDs

Multiplicity is common in field stars and among protostellar systems. Models suggest two paths of formation: turbulent fragmentation and protostellar disk fragmentation. We attempt to find whether or not the coevality frequency of multiple protostellar systems can help to better understand their formation mechanism. The coevality frequency is determined by constraining the relative evolutionary stages of the components in a multiple system. SEDs for known multiple protostars in Perseus were constructed from literature data. Herschel PACS photometric maps were used to sample the peak of the SED for systems with separations >7", a crucial aspect in determining the evolutionary stage of a protostellar system. Inclination effects and the surrounding envelope and outflows were considered to decouple source geometry from evolution. This together with the shape and derived properties from the SED was used to determine each system's coevality as accurately as possible. SED models were used to examine the frequency of non-coevality that is due to geometry. We find a non-coevality frequency of 33+/-10% from the comparison of SED shapes of resolved multiple systems. Other source parameters suggest a somewhat lower frequency of non-coevality. The frequency of apparent non-coevality that is due to random inclination angle pairings of model SEDs is 17+/-0.5%. Observations of the outflow of resolved multiple systems do not suggest significant misalignments within multiple systems. Effects of unresolved multiples on the SED shape are also investigated. We find that 1/3 of the multiple protostellar systems sampled here are non-coeval, which is more than expected from random geometric orientations. The other 2/3 are found to be coeval. Higher order multiples show a tendency to be non-coeval. The frequency of non-coevality found here is most likely due to formation and enhanced by dynamical evolution.

astro-ph.SR↗

arXiv2015

A low-mass protostar's disk-envelope interface: disk-shadowing evidence from ALMA DCO+ observations of VLA1623

Due to instrumental limitations and a lack of disk detections, the structure between the envelope and the rotationally supported disk has been poorly studied. This is now possible with ALMA through observations of CO isotopologs and tracers of freezeout. Class 0 sources are ideal for such studies given their almost intact envelope and young disk. The structure of the disk-envelope interface of the prototypical Class 0 source, VLA1623A which has a confirmed Keplerian disk, is constrained from ALMA observations of DCO+ 3-2 and C18O 2-1. The physical structure of VLA1623 is obtained from the large-scale SED and continuum radiative transfer. An analytic model using a simple network coupled with radial density and temperature profiles is used as input for a 2D line radiative transfer calculation for comparison with the ALMA Cycle 0 12m array and Cycle 2 ACA observations of VLA1623. DCO+ emission shows a clumpy structure bordering VLA1623A's Keplerian disk, suggesting a cold ring-like structure at the disk-envelope interface. The radial position of the observed DCO+ peak is reproduced in our model only if the region's temperature is between 11-16K, lower than expected from models constrained by continuum and SED. Altering the density has little effect on the DCO+ position, but increased density is needed to reproduce the disk traced in C18O. The DCO+ emission around VLA1623A is the product of shadowing of the envelope by the disk. Disk-shadowing causes a drop in the gas temperature outside of the disk on >200AU scales, encouraging deuterated molecule production. This indicates that the physical structure of the disk-envelope interface differs from the rest of the envelope, highlighting the drastic impact that the disk has on the envelope and temperature structure. The results presented here show that DCO+ is an excellent cold temperature tracer.

astro-ph.SR↗

arXiv2013

A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A

Context: Rotationally supported disks are critical in the star formation process. The questions of when do they form and what factors influence or hinder their formation have been studied but are largely unanswered. Observations of early stage YSOs are needed to probe disk formation. Aims: VLA1623 is a triple non-coeval protostellar system, with a weak magnetic field perpendicular to the outflow, whose Class 0 component, VLA1623A, shows a disk-like structure in continuum with signatures of rotation in line emission. We aim to determine whether this structure is in part or in whole a rotationally supported disk, i.e. a Keplerian disk, and what are its characteristics. Methods: ALMA Cycle 0 Early Science 1.3 mm continuum and C$^{18}$O (2-1) observations in the extended configuration are presented here and used to perform an analysis of the disk-like structure using PV diagrams and thin disk modelling with the addition of foreground absorption. Results: The PV diagrams of the C$^{18}$O line emission suggest the presence of a rotationally supported component with a radius of at least 50 AU. Kinematical modelling of the line emission shows that the disk out to 180 AU is actually rotationally supported, with the rotation being well described by Keplerian rotation out to at least 150 AU, and the central source mass to be $\sim$0.2 M$_{sun}$ for an inclination of 55$^{\circ}$. Pure infall and conserved angular momentum rotation models are excluded. Conclusions: VLA1623A, a very young Class 0 source, presents a disk with an outer radius $R_{\rm out}$ = 180 AU with a Keplerian velocity structure out to at least 150 AU. The weak magnetic fields and recent fragmentation in this region of rho Ophiuchus may have played a lead role in the formation of the disk.

astro-ph.GA↗

arXiv2013

Disentangling the entangled: Observations and analysis of the triple non-coeval protostellar system VLA1623

Commonplace at every evolutionary stage, Multiple Protostellar Systems (MPSs) are thought to be formed through fragmentation, but it is unclear when and how. The youngest MPSs, which have not yet undergone much evolution, provide important constraints to this question. It is then of interest to disentangle early stage MPSs. In this letter we present the results of our work on VLA1623 using our observations and archival data from the Submillimeter Array (SMA). Our continuum and line observations trace VLA1623's components, outflow and envelope, revealing unexpected characteristics. We construct the SED for each component using the results of our work and data from literature, as well as derive physical parameters from continuum and perform a simple kinematical analysis of the circumstellar material. Our results show VLA1623 to be a triple non-coeval system composed of VLA1623A, B & W, with each source driving its own outflow and unevenly distributed circumstellar material. From the SED, physical parameters and IR emission we conclude that VLA1623A & W are Class 0 and I protostars, respectively, and together drive the bulk of the observed outflow. Furthermore, we find two surprising results, first the presence of a rotating disk-like structure about VLA1623A with indications of pure Keplerian rotation, which, if real, would make it one of the first evidence of Keplerian disk structures around Class 0 protostars. Second, we find VLA1623B to be a bonafide extremely young protostellar object between the starless core and Class 0 stages.

astro-ph.GA↗