arXiv subjects

Aina Palau

Publications and source records attributed to Aina Palau.

At least 55 records · Page 3Linked to original sources

arXiv2021

Does the magnetic field suppress fragmentation in massive dense cores?

Theoretical and numerical works indicate that a strong magnetic field should suppress fragmentation in dense cores. However, this has never been tested observationally in a relatively large sample of fragmenting massive dense cores. Here we use the polarization data obtained in the Submillimeter Array Legacy Survey of Zhang et al. to build a sample of 18 massive dense cores where both fragmentation and magnetic field properties are studied in a uniform way. We measured the fragmentation level, Nmm, within the field of view common to all regions, of 0.15 pc, with a mass sensitivity of about 0.5 Msun, and a spatial resolution of about 1000 au. In order to obtain the magnetic field strength using the Davis-Chandrasekhar-Fermi method, we estimated the dispersion of the polarization position angles, the velocity dispersion of the H13CO+(4-3) gas, and the density of each core, all averaged within 0.15 pc. A strong correlation is found between Nmm and the average density of the parental core, although with significant scatter. When large-scale systematic motions are separated from the velocity dispersion and only the small-scale (turbulent) contribution is taken into account, a tentative correlation is found between Nmm and the mass-to-flux ratio, as suggested by numerical and theoretical works.

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arXiv2021

Density profile evolution during prestellar core collapse: Collapse starts at the large scale

We study the gravitationally-dominated, accretion-driven evolution of a prestellar core. In our model, as the core's density increases, it remains immersed in a constant-density environment and so it accretes from this environment, increasing its mass and reducing its Jeans length. Assuming a power-law density profile $ρ\propto r^{-p}$, we compute the rate of change of the slope $p$, and show that the value $p=2$ is stationary, and furthermore, an attractor. The radial profile of the Jeans length scales as $r^{p/2}$, implying that, for $p<2$, there is a radius below which the region is smaller than its Jeans length, thus appearing gravitationally stable and in need of pressure confinement, while, in reality, it is part of a larger-scale collapse and is undergoing compression by the infalling material. In this region, the infall speed decreases towards the center, eventually becoming subsonic, thus appearing "coherent", without the need for turbulence dissipation. We present a compilation of observational determinations of density profiles in dense cores and show that the distribution of their slopes peaks at $p \sim 1.7$--1.9, supporting the notion that the profile steepens over time. Finally, we discuss the case of magnetic support in a core in which the field scales as $B \propto ρ^β$. For the expected value of $β= 2/3$, this implies that the mass to magnetic flux ratio also decreases towards the central parts of the cores, making them appear magnetically supported, while in reality they may be part of larger collapsing supercritical region. We conclude that local signatures of either thermal or magnetic support are not conclusive evidence of stability, that the gravitational instability of a region must be established at the large scales, and that the prestellar stage of collapse is dynamic rather than quasistatic.

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arXiv2020

VLA ammonia observations of L1287: analysis of the "Guitar" core and two filaments

The present work aims at studying the dense gas of the molecular cloud LDN 1287 (L1287), which harbors a double FU Ori system, an energetic molecular outflow and a still-forming cluster of deeply embedded low-mass, young stellar objects, showing a high level of fragmentation. We present optical Halpha and [SII], and VLA NH$_3$ (1,1) and (2,2) observations with an angular resolution of ~3.5''. The observed NH$_3$ spectra have been analyzed with the HfS tool, fitting simultaneously three different velocity components. The NH$_3$ emission from L1287 comes from four different structures: a core associated with RNO 1, a guitar-shaped core (the "Guitar") and two interlaced filaments (the Blue and Red Filaments) roughly centered towards the binary FU Ori system RNO 1B/C and its associated cluster. Regarding the Guitar Core, there are clear signatures of gas infall onto a central mass that has been estimated to be ~2.1 $M_\odot$. Regarding the two filaments, they have radii ~0.03 pc, masses per unit length ~50 $M_\odot$ pc$^{-1}$, and are near isothermal equilibrium. A central cavity, probably related with the outflow, and also traced by the Halpha and [SII] emission, is identified, with several young stellar objects near its inner walls. Both filaments show clear signs of perturbation by the high-velocity gas of the outflows driven by one or several young stellar objects of the cluster. The Blue and Red filaments are coherent in velocity and have nearly subsonic gas motions, except at the position of the embedded sources. Velocity gradients across the Blue Filament can be interpreted either as infalling material onto the filament or rotation. Velocity gradients along the filaments are interpreted as infall motions towards a gravitational well at the intersection of both filaments.

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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

Simultaneous evolution of the virial parameter and star formation rate in molecular clumps undergoing global hierarchical collapse

We compare dense clumps and cores in a numerical simulation of molecular clouds (MCs) undergoing global hierarchical collapse (GHC) to observations in two MCs at different evolutionary stages, the Pipe and the G14.225 clouds, to test the ability of the GHC scenario to follow the early evolution of the energy budget and star formation activity of these structures. In the simulation, we select a region that contains cores of sizes and densities similar to the Pipe cores, and find that it evolves through accretion, developing substructure similar to that of G14.225 cloud after $\sim 1.6$ Myr. Within this region, we follow the evolution of the Larson ratio $\mathcal{L} \equiv σ_{\rm v}/R^{1/2}$, where $σ_{\rm v}$ is the velocity dispersion and $R$ is the size, the virial parameter $α$, and the star formation activity of the cores/clumps. In the simulation, we find that as the region evolves: $i)$ its clumps have $\mathcal{L}$ and $α$ values first consistent with those of the Pipe substructures and later with those of G14.225; $ii)$ the individual cores first exhibit a decrease in $α$ followed by an increase when star formation begins; $iii)$ collectively, the ensemble of cores/clumps reproduces the observed trend of lower $α$ for higher-mass objects, and $iv)$ the star formation rate and star formation efficiency increase monotonically. We suggest that this evolution is due to the simultaneous loss of externally-driven compressive kinetic energy and increase of the self-gravity-driven motions. We conclude that the GHC scenario provides a realistic description of the evolution of the energy budget of the clouds' substructure at early times, which occurs simultaneously with an evolution of the star formation activity.

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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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arXiv2020

ALMA observations of NGC 6334S $-$ I: Forming massive stars and cluster in subsonic and transonic filamentary clouds

We present Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) observations of the massive infrared dark cloud NGC 6334S (also known as IRDC G350.56+0.44), located at the southwestern end of the NGC 6334 molecular cloud complex. The H$^{13}$CO$^{+}$ and the NH$_{2}$D lines covered by the ALMA observations at a $\sim$3$^{\prime\prime}$ angular resolution ($\sim$0.02 pc) reveal that the spatially unresolved non-thermal motions are predominantly subsonic and transonic, a condition analogous to that found in low-mass star-forming molecular clouds. The observed supersonic non-thermal velocity dispersions in massive star forming regions, often reported in the literature, might be significantly biased by poor spatial resolutions that broaden the observed line widths due to unresolved motions within the telescope beam. Our 3~mm continuum image resolves 49 dense cores, whose masses range from 0.17 to 14 $M_{\odot}$. The majority of them are resolved with multiple velocity components. Our analyses of these gas velocity components find an anti-correlation between the gas mass and the virial parameter. This implies that the more massive structures tend to be more gravitationally unstable. Finally, we find that the external pressure in the NGC 6334S cloud is important in confining these dense structures, and may play a role in the formation of dense cores, and subsequently, the embedded young stars.

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arXiv2019

Global Hierarchical Collapse In Molecular Clouds. Towards a Comprehensive Scenario

We present a unified description of the scenario of Global Hierarchical Collapse and fragmentation (GHC) in molecular clouds (MCs), owing to the continuous decrease of the average Jeans mass in the contracting cloud. GHC constitutes a regime of collapses within collapses, in which small-scale collapses begin at later times, but occur on shorter timescales than large-scale ones. The difference in timescales allows for most of the clouds' mass to be dispersed by feedback from the first massive stars, maintaining the global star formation rate low. All scales accrete from their parent structures. The main features of GHC are: star-forming MCs are in an essentially pressureless regime, which produces filaments that accrete onto clumps and cores ("hubs"). The filaments constitute the collapse flow from cloud to hub scales and may approach a quasi-stationary state; the molecular and dense mass fractions of the clouds increase over time; the first (low-mass) stars appear several Myr after global contraction began; more massive stars appear after a few Myr in massive hubs resulting from the collapse of larger scales; the minimum fragment mass may extend well into the brown-dwarf regime; Bondi-Hoyle-Lyttleton accretion occurs at the protostellar and core scales, accounting for a near-Salpeter IMF; the extreme anisotropy of the filamentary network explains the difficulty in detecting large-scale infall signatures; the balance between inertial and gravitationally-driven motions in clumps evolves during the contraction; prestellar cores adopt Bonnor-Ebert-like profiles, but are contracting ever since early times when they may appear to be unbound and to require pressure confinement; stellar clusters develop radial age and mass segregation gradients. Finally, we discuss the incompatibility between supersonic turbulence and the observed scalings in the molecular hierarchy.

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arXiv2019

Not so different after all: Properties and Spatial Structure of Column Density Peaks in the Pipe and Orion A Clouds

We present a comparative study of the physical properties and the spatial distribution of column density peaks in two Giant Molecular Clouds (GMC), the Pipe Nebula and Orion A, which exemplify opposite cases of star cluster formation stages. The density peaks were extracted from dust extinction maps constructed from Herschel/SPIRE farinfrared images. We compare the distribution functions for dust temperature, mass, equivalent radius and mean volume density of peaks in both clouds, and made a more fair comparison by isolating the less active Tail region in Orion A and by convolving the Pipe Nebula map to simulate placing it at a distance similar to that of the Orion Complex. The peak mass distributions for Orion A, the Tail, and the convolved Pipe, have similar ranges, sharing a maximum near 5 M$_\odot$, and a similar power law drop above 10 M$_\odot$. Despite the clearly distinct evolutive stage of the clouds, there are very important similarities in the physical and spatial distribution properties of the column density peaks, pointing to a scenario where they form as a result of uniform fragmentation of filamentary structures across the various scales of the cloud, with density being the parameter leading the fragmentation, and with clustering being a direct result of thermal fragmentation at different spatial scales. Our work strongly supports the idea that the formation of clusters in GMC could be the result of the primordial organization of pre-stellar material

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arXiv2019

G5.89: An Explosive Outflow Powered by a Proto-Stellar Merger?

The explosive outflows are a newly-discovered family of molecular outflows associated with high-mass star forming regions. Such energetic events are possibly powered by the release of gravitational energy related with the formation of a (proto)stellar merger or a close stellar binary. Here, we present sensitive and high angular resolution observations (0.85$''$) archival CO(J=3-2) observations carried out with the Submillimeter Array (SMA) of the high-mass star forming region G5.89$-$0.39 that reveal the possible presence of an explosive outflow. We find six well-defined and narrow straight filament-like ejections pointing back approximately to the center of an expanding molecular and ionized shell located at the center of this region. These high velocity ($-$120 to $+$100 km s$^{-1}$) filaments follow a Hubble-like velocity law with the radial velocities increasing with the projected distance. The estimated kinematical age of the filaments is about of 1000 yrs, a value similar to the dynamical age found for the expanding ionized shell. G5.89 is the thus the third explosive outflow reported in the galaxy (together with Orion BN-KL and DR21) and argues in favor of the idea that this is a frequent phenomenon. In particular, explosive outflows, in conjunction with runaway stars, demonstrate that dynamical interactions in such groups are a very important ingredient in star formation.

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arXiv2019

Filamentary Accretion Flows in the Infrared Dark Cloud G14.225-0.506 Revealed by ALMA

Filaments are ubiquitous structures in molecular clouds and play an important role in the mass assembly of stars. We present results of dynamical stability analyses for filaments in the infrared dark cloud G14.225$-$0.506, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. Full-synthesis imaging is performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to map the $\mathrm{N_2H^+} \; (1-0)$ emission in two hub-filament systems with a spatial resolution of $\sim 0.034 \; \mathrm{pc}$. Kinematics are derived from sophisticated spectral fitting algorithm that accounts for line blending, large optical depth, and multiple velocity components. We identify five velocity coherent filaments and derive their velocity gradients with principal component analysis. The mass accretion rates along the filaments are up to $10^{-4} \; \mathrm{M_\odot \, \mathrm{yr^{-1}}}$ and are significant enough to affect the hub dynamics within one free-fall time ($\sim 10^5 \; \mathrm{yr}$). The $\mathrm{N_2H^+}$ filaments are in equilibrium with virial parameter $α_\mathrm{vir} \sim 1.2$. We compare $α_\mathrm{vir}$ measured in the $\mathrm{N_2H^+}$ filaments, $\mathrm{NH_3}$ filaments, $870 \; μ\mathrm{m}$ dense clumps, and $3 \; \mathrm{mm}$ dense cores. The decreasing trend in $α_\mathrm{vir}$ with decreasing spatial scales persists, suggesting an increasingly important role of gravity at small scales. Meanwhile, $α_\mathrm{vir}$ also decreases with decreasing non-thermal motions. In combination with the absence of high-mass protostars and massive cores, our results are consistent with the global hierarchical collapse scenario.

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arXiv2019

An Asymmetric Keplerian Disk Surrounding the O-type Protostar IRAS16547$-$4247

During the last decades, a great interest has emerged to know if even the most massive stars in our galaxy (namely the spectral O-type stars) are formed in a similar manner as the low- and intermediate-mass stars, that is, through the presence of accreting disks and powerful outflows. Here, using sensitive observations of the Atacama Large Millimeter/Submillimeter Array (ALMA), we report a resolved Keplerian disk (with fifteen synthesized beams across its major axis) surrounding the deeply embedded O-type protostar IRAS16547$-$4247. The disk shows some asymmetries that could arise because of the disk is unstable and fragmenting or maybe because of different excitation conditions within the disk. The enclosed mass estimated from the disk Keplerian radial velocities is 25$\pm$3 M$_\odot$. The molecular disk is at the base of an ionized thermal radio jet and is approximately perpendicular to the jet axis orientation. We additionally find the existence of a binary system of compact dusty objects at the center of the accreting disk, which indicates the possible formation of an O-type star and a companion of lower mass. This is not surprising due to the high binary fraction reported in massive stars. Subtracting the contribution of the dusty disk plus the envelope and the companion, we estimated a mass of 20 M$_\odot$ for the central star.

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arXiv2018

Extreme fragmentation and complex kinematics at the center of the L1287 cloud

The filamentary infrared dark cloud L1287 is actively forming a dense cluster of low-mass YSOs at its inner $\sim$0.1 pc region. To help understand the origin of this low-mass YSO cluster, the present work aims at resolving the gas structures and kinematics. We have performed $\sim$1$"$ angular resolution ($\sim$930 AU) SMA observations at $\sim$1.3 mm. From a $\sim$2$"$ resolution 1.3 mm continuum image we identified six dense cores, namely SMA1-6 with masses in the range of $\sim0.4-4$ M$_\odot$. From a $\sim$1$"$ resolution 1.3 mm continuum image, we find a high fragmentation level, with 14 compact millimeter sources within 0.1 pc (two of them associated with the known accretion outburst YSOs RNO 1C and RNO 1B). The dense gas tracer DCN (3--2) traces well the dust continuum emission and shows a clear velocity gradient along the NW-SE direction centered at SMA3. There is another velocity gradient with opposite direction around the most luminous YSO IRAS 00338+6312. The fragmentation within 0.1 pc in L1287 is very high compared to other regions at the same spatial scales. The incoherent motions of dense gas flows are sometimes interpreted by being influenced by (proto)stellar feedback (e.g., outflows), which is not yet ruled out in this particular target source. The directions of the velocity gradients traced by DCN are approximately perpendicular to those of the dominant CO outflow(s). Therefore, we alternatively hypothesize that the velocity gradients revealed by DCN trace the convergence from the $\gtrsim$0.1 pc scales infalling motion towards the rotational motions around the more compact ($\sim0.02$ pc) sources. This global molecular gas converging flow may feed the formation of the dense low-mass YSO cluster. IRAS 00338+6312 is the most likely powering source of the dominant CO outflow. A compact blue-shifted outflow from RNO 1C is also identified.

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arXiv2018

Thermal Jeans fragmentation within 1000 AU in OMC-1S

We present subarcsecond 1.3 mm continuum ALMA observations towards the Orion Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 AU, which reveal a total of 31 continuum sources. We also present subarcsecond 7 mm continuum VLA observations of the same region, which allow to further study fragmentation down to a spatial resolution of 40 AU. By applying a Mean Surface Density of Companions method we find a characteristic spatial scale at ~560 AU, and we use this spatial scale to define the boundary of 19 `cores' in OMC-1S as groupings of millimeter sources. We find an additional characteristic spatial scale at ~2900 AU, which is the typical scale of the filaments in OMC-1S, suggesting a two-level fragmentation process. We measured the fragmentation level within each core and find a higher fragmentation towards the southern filament. In addition, the cores of the southern filament are also the densest (within 1100 AU) cores in OMC-1S. This is fully consistent with previous studies of fragmentation at spatial scales one order of magnitude larger, and suggests that fragmentation down to 40 AU seems to be governed by thermal Jeans processes in OMC-1S.

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arXiv2017

A 1.3 mm SMA Survey of 29 Variable Young Stellar Objects

Young stellar objects (YSOs) may undergo periods of active accretion (outbursts), during which the protostellar accretion rate is temporarily enhanced by a few orders of magnitude. Whether or not these accretion outburst YSOs possess similar dust/gas reservoirs to each other, and whether or not their dust/gas reservoirs are similar as quiescent YSOs, are issues not yet clarified. The aim of this work is to characterize the millimeter thermal dust emission properties of a statistically significant sample of long and short duration accretion outburst YSOs (i.e., FUors and EXors) and the spectroscopically identified candidates of accretion outbursting YSOs (i.e., FUor-like objects). We have carried out extensive Submillimeter Array (SMA) observations mostly at $\sim$225 GHz (1.33 mm) and $\sim$272 GHz (1.10 mm), from 2008 to 2017. We covered accretion outburst YSOs located at $<$1 kpc distances from the solar system. We analyze all the existing SMA data of such objects, both published and unpublished, in a coherent way to present a millimeter interferometric database of 29 objects. We obtained 21 detections at $>$3-$σ$ significance. Detected sources except for the two cases of V883 Ori and NGC 2071 MM3 were observed with $\sim$1$"$ angular resolution. Overall our observed targets show a systematically higher millimeter luminosity distribution than those of the $M_{*}>$0.3 $M_{\odot}$ Class II YSOs in the nearby ($\lesssim$400 pc) low-mass star-forming molecular clouds (e.g., Taurus, Lupus, Upp Scorpio, and Chameleon I). In addition, at 1 mm our observed confirmed binaries or triple-system sources are systematically fainter than the rest of the sources even though their 1 mm fluxes are broadly distributed. We may have detected $\sim$30-60\% millimeter flux variability from V2494 Cyg and V2495 Cyg, from the observations separated by $\sim$1 year.

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arXiv2017

Gravity or turbulence? IV. Collapsing cores in out-of-virial disguise

We study the dynamical state of cores by using a simple analytical model, a sample of observational massive cores, and numerical simulations of collapsing massive cores. From the model, we find that, if cores are formed from turbulent compressions, they evolve from small to large column densities, increasing their velocity dispersion as they collapse, while they tend to equipartition between gravity and kinetic energy. From the observational sample, we find that: (a) cores with substantially different column densities in the sample do not follow a Larson-like linewidth-size relation. Instead, cores with higher column densities tend to be located in the upper-left corner of the Larson velocity dispersion-size diagram, a result predicted previously (Ballesteros-Paredes et al. 2011a). (b) The data exhibit cores with overvirial values. Finally, in the simulations we reproduce the behavior depicted by the model and observational sample: cores evolve towards larger velocity dispersions and smaller sizes as they collapse and increase their column density. However, collapsing cores appear to approach overvirial states within a free-fall time. The cause of this apparent excess of kinetic energy is an underestimation of the gravitational energy, due to the assumption that the gravitational energy is given by the energy of an isolated sphere of constant column density. This excess disappears when the gravitational energy is correctly calculated from the actual spatial mass distribution, where inhomogeneities, as well as the potential due to the mass outside of the core, also contribute to the gravitational energy. We conclude that the observed energy budget of cores in surveys is consistent with their non-thermal motions being driven by their self-gravity and in a hierarchical and chaotic collapse.

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arXiv2017

Magnetized converging flows towards the hot core in the intermediate/high-mass star-forming region NGC 6334 V

We present Submillimeter Array (SMA) observations at 345 GHz towards the intermediate/high-mass cluster-forming region NGC 6334 V. From the dust emission we spatially resolve three dense condensations, the brightest one presenting the typical chemistry of a hot core. The magnetic field (derived from the dust polarized emission) shows a bimodal converging pattern towards the hot core. The molecular emission traces two filamentary structures at two different velocities, separated by 2 km/s, converging to the hot core and following the magnetic field distribution. We compare the velocity field and the magnetic field derived from the SMA observations with MHD simulations of star-forming regions dominated by gravity. This comparison allows us to show how the gas falls in from the larger-scale extended dense core (~0.1 pc) of NGC 6334 V towards the higher-density hot core region (~0.02 pc) through two distinctive converging flows dragging the magnetic field, whose strength seems to have been overcome by gravity.

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arXiv2017

A search for water maser emission from brown dwarfs and low-luminosity young stellar objects

We present a survey for water maser emission toward a sample of 44 low-luminosity young objects, comprising (proto-)brown dwarfs, first hydrostatic cores (FHCs), and other young stellar objects (YSOs) with bolometric luminosities lower than 0.4 L$_\odot$. Water maser emission is a good tracer of energetic processes, such as mass-loss and/or accretion, and is a useful tool to study this processes with very high angular resolution. This type of emission has been confirmed in objects with L$_{\rm bol}\ge 1$ L$_\odot$. Objects with lower luminosities also undergo mass-loss and accretion, and thus, are prospective sites of maser emission. Our sensitive single-dish observations provided a single detection when pointing toward the FHC L1448 IRS 2E. However, follow-up interferometric observations showed water maser emission associated with the nearby YSO L1448 IRS 2 { (a Class 0 protostar of L$_{\rm bol}\simeq 3.6-5.3$ L$_\odot$)}, and did not find any emission toward L1448 IRS 2E. The upper limits for water maser emission determined by our observations are one order of magnitude lower than expected from the correlation between water maser luminosities and bolometric luminosities found for YSOs. This suggests that this correlation does not hold at the lower end of the (sub)stellar mass spectrum. Possible reasons are that the slope of this correlation is steeper at L$_{\rm bol}\le 1$ L$_\odot$, or that there is an absolute luminosity threshold below which water maser emission cannot be produced. Alternatively, if the correlation still stands at low luminosity, the detection rates of masers would be significantly lower than the values obtained in higher-luminosity Class 0 protostars.

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