Imaging the water snowline in protostellar envelopes

Imaging the water snowline in protostellar envelopes

Author:

van 't Hoff

Abstract:

Determining the locations of the major snowlines in protostellar environments is crucial to fully understand the planet formation process and its outcome. Despite being located far enough from the central star to be spatially resolved with ALMA, the CO snowline remains difficult to detect directly in protoplanetary disks. Instead, its location can be derived from N2H+ emission, when chemical effects like photodissociation of CO and N2 are taken into account. The water snowline is even harder to observe than that for CO, because in disks it is located only a few AU from the protostar, and from the ground only the less abundant isotopologue H182O can be observed. Therefore, using an indirect chemical tracer, as done for CO, may be the best way to locate the water snowline. A good candidate tracer is HCO+, which is expected to be particularly abundant when its main destructor, H2O, is frozen out. Comparison of H182O and H13CO+ emission toward the envelope of the Class 0 protostar IRAS2A shows that the emission from both molecules is spatially anticorrelated, providing a proof of concept that H13CO+ can indeed be used to trace the water snowline in systems where it cannot be imaged directly.

Decoupling of Magnetic Fields in Collapsing Protostellar Envelopes and Disk Formation and Fragmentation

Decoupling of Magnetic Fields in Collapsing Protostellar Envelopes and Disk Formation and Fragmentation

Authors:

Zhao et al

Abstract:
Efficient magnetic braking is a formidable obstacle to the formation of rotationally supported disks (RSDs) around protostars in magnetized dense cores. We have previously shown, through 2D (axisymmetric) non-ideal MHD simulations, that removing very small grains (VSGs: ~10 \AA to few 100 \AA) can greatly enhance ambipolar diffusion and enable the formation of RSDs. Here we extend the simulations of disk formation enabled by VSG removal to 3D. We find that the key to this scenario of disk formation is that the infall speed of ions (and thus the magnetic field lines tied to them) almost vanishes in the 102-103 AU-scale "pseudo-disk" where the field lines are most severely pinched and most of protostellar envelope mass infall occurs. As a result, the bulk neutral envelope matter can collapse without dragging much magnetic flux into the disk-forming region, which lowers the magnetic braking efficiency. We find that the initial disks enabled by VSG removal tend to be Toomre-unstable, which leads to the formation of prominent spiral structures that function as centrifugal barriers. The piling-up of infall material near the centrifugal barrier often produces dense fragments of tens of Jupiter masses, especially in cores that are not too strongly magnetized. Some fragments accrete onto the central stellar object, producing bursts in mass accretion rate. Others are longer lived, although whether they can survive long-term to produce multiple systems remains to be ascertained. Our results highlight the importance of dust grain evolution in determining the formation and properties of protostellar disks and potentially multiple systems.

G11.92–0.61 MM1: A Keplerian disc around a massive young proto-O star

G11.92–0.61 MM1: A Keplerian disc around a massive young proto-O star

Authors:

Ilee et al

Abstract:

The formation process of massive stars is not well understood, and advancement in our understanding benefits from high resolution observations and modelling of the gas and dust surrounding individual high-mass (proto)stars. Here we report sub-arcsecond (≲1550 au) resolution observations of the young massive star G11.92–0.61 MM1 with the SMA and VLA. Our 1.3 mm SMA observations reveal consistent velocity gradients in compact molecular line emission from species such as CH3CN, CH3OH, OCS, HNCO, H2CO, DCN and CH3CH2CN, oriented perpendicular to the previously-reported bipolar molecular outflow from MM1. Modelling of the compact gas kinematics suggests a structure undergoing rotation around the peak of the dust continuum emission. The rotational profile can be well fit by a model of a Keplerian disc, including infall, surrounding an enclosed mass of ∼30–60 M⊙, of which 2–3 M⊙ is attributed to the disc. From modelling the CH3CN emission, we determine that two temperature components, of ∼ 150 K and 230 K, are required to adequately reproduce the spectra. Our 0.9 and 3.0 cm VLA continuum data exhibit an excess above the level expected from dust emission; the full centimetre-submillimetre wavelength spectral energy distribution of MM1 is well reproduced by a model including dust emission, an unresolved hypercompact H ii region, and a compact ionised jet. In combination, our results suggest that MM1 is an example of a massive proto-O star forming via disc accretion, in a similar way to that of lower mass stars.

ROTATIONALLY DRIVEN FRAGMENTATION IN THE FORMATION OF THE BINARY PROTOSTELLAR SYSTEM L1551 IRS 5

ROTATIONALLY DRIVEN FRAGMENTATION IN THE FORMATION OF THE BINARY PROTOSTELLAR SYSTEM L1551 IRS 5

Authors:

Lim et al

Abstract:

Both bulk rotation and local turbulence have been widely suggested to drive the fragmentation in collapsing cores that produces multiple star systems. Even when the two mechanisms predict different alignments for stellar spins and orbits, subsequent internal or external interactions can drive multiple systems toward or away from alignment, thus masking their formation processes. Here, we demonstrate that the geometrical and dynamical relationship between a binary system and its surrounding bulk envelope provide the crucial distinction between fragmentation models. We find that the circumstellar disks of the binary protostellar system L1551 IRS 5 are closely parallel, not just with each other but also with their surrounding flattened envelope. Measurements of the relative proper motion of the binary components spanning nearly 30 years indicate an orbital motion related to that of the envelope rotation. Eliminating orbital solutions whereby the circumstellar disks would be tidally truncated to sizes smaller than observed, the remaining solutions favor a circular or low-eccentricity orbit tilted by up to ~25° from the circumstellar disks. Turbulence-driven fragmentation can generate local angular momentum to produce a coplanar binary system, but this would have no particular relationship to the system's surrounding envelope. Instead, the observed properties conform with predictions for rotationally driven fragmentation. If the fragments were produced at different heights or on opposite sides of the mid-plane in the flattened central region of a rotating core, the resulting protostars would then exhibit circumstellar disks parallel with the surrounding envelope but tilted from the orbital plane, as is observed.

Evidence of Magnetic Braking in the HH 111 Protostellar System?

ANGULAR MOMENTUM LOSS IN THE ENVELOPE–DISK TRANSITION REGION OF THE HH 111 PROTOSTELLAR SYSTEM: EVIDENCE FOR MAGNETIC BRAKING?

Authors:

Lee et al

Abstract:

HH 111 is a Class I protostellar system at a distance of ~400 pc, with the central source VLA 1 associated with a rotating disk deeply embedded in a flattened envelope. Here we present the observations of this system at ~0farcs6 (240 au) resolution in C18O (J = 2 — 1) and a 230 GHz continuum obtained with the Atacama Large Millimeter/Submillimeter Array, and in SO (${N}_{J}\,=\,{5}_{6}-{4}_{5}$) obtained with the Submillimeter Array. The observations show for the first time how a Keplerian rotating disk can be formed inside a flattened envelope. The flattened envelope is detected in C18O, extending out to gsim2400 au from the VLA 1 source. It has a differential rotation, with the outer part (gsim2000 au) better described by a rotation that has constant specific angular momentum, and the innermost part (lesssim160 au) by a Keplerian rotation. The rotationally supported disk is therefore relatively compact in this system, which is consistent with the dust continuum observations. Most interestingly, if the flow is in steady state, there is a substantial drop in specific angular momentum in the envelope–disk transition region from 2000 to 160 au, by a factor of ~3. Such a decrease is not expected outside a disk formed from simple hydrodynamic core collapse, but can happen naturally if the core is significantly magnetized, because magnetic fields can be trapped in the transition region outside the disk by the ram pressure of the protostellar accretion flow, which can lead to efficient magnetic braking. In addition, SO shock emission is detected around the outer radius of the disk and could trace an accretion shock around the disk.

The Snowline is NOT Static Over the Life of a Star

Imaging the water-snow line during a protostellar outburst

Authors:

Cieza et al

Abstract:

A snow-line is the region of a protoplanetary disk at which a major volatile, such as water or carbon monoxide, reaches its condensation temperature. Snow-lines play a crucial role in disk evolution by promoting the rapid growth of ice-covered grains. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged in the disks surrounding the pre-main-sequence stars TW Hydra and HD163296, at distances of about 30 astronomical units (au) from the star. But the water snow-line of a protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto been seen, as it generally lies very close to the star (less than 5 au away for solar-type stars). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation, and the formation of comets, ice giants and the cores of gas giants. Here we report ALMA images at 0.03-arcsec resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an outburst in luminosity arising from a temporary increase in the accretion rate. We find an intensity break corresponding to an abrupt change in the optical depth at about 42 au, where the elevated disk temperature approaches the condensation point of water, from which we conclude that the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions: dust fragmentation and the inhibition of grain growth at higher temperatures results in soaring grain number densities and optical depths. As most planetary systems are expected to experience outbursts caused by accretion during their formation our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation.

Ejection of gaseous clumps from gravitationally unstable protostellar disks

Ejection of gaseous clumps from gravitationally unstable protostellar disks

Authors:

Vorobyov et al

Abstract:

We investigate the dynamics of gaseous clumps formed via gravitational fragmentation in young protostellar disks, focusing on the fragments that are ejected from the disk via many-body gravitational interaction. Numerical hydrodynamics simulations were employed to study the evolution of young protostellar disks formed from the collapse of rotating pre-stellar cores with mass in the 1.1-1.6 M_sun range. Protostellar disks formed in our models undergo gravitational fragmentation driven by continuing mass loading from parental collapsing cores. A few fragments can be ejected from the disk during the early evolution, but the low-mass fragments (less than 15~M_Jup) disperse creating spectacular bow-type structures while passing through the disk and collapsing core. The least massive fragment that survived the ejection (21 M_Jup) straddles the planetary-mass limit, while the most massive ejected fragments (145 M_Jup) can break up into several pieces, leading to the ejection of wide separation binary clumps in the brown-dwarf mass range. About half of the ejected fragments are gravitationally bound, the majority is supported by rotation against gravity, and all fragments have the specific angular momentum that is much higher than that expected for brown dwarfs. We found that the internal structure of the ejected fragments is distinct from what would be expected for gravitationally contracting clumps formed via cloud core fragmentation, which can help to differentiate their origin. The ejection of fragments is an important process inherent to massive protostellar disks, which produces freely-floating pre-brown dwarf cores, regulates the disk and stellar masses, and potentially enriches the intracluster medium with processed dust and complex organics.

Signatures of Gravitational Instability in Resolved Images of Protostellar Disks

Signatures of Gravitational Instability in Resolved Images of Protostellar Disks

Authors:

Dong et al

Abstract:

Protostellar (class 0/I) disks, having masses comparable to those of their nascent host stars, and fed continuously from their natal infalling envelopes, are prone to gravitational instability (GI). Motivated by advances in near-infrared (NIR) adaptive optics imaging and mm-wave interferometry, we explore the observational signatures of GI in disks, using hydrodynamical and Monte Carlo radiative transfer simulations to synthesize NIR scattered light images and mm dust continuum maps. Spiral arms induced by GI, located at disk radii of hundreds of AUs, are local overdensities and have their photospheres displaced to higher altitudes above the disk midplane, arms therefore scatter more NIR light from their central stars than inter-arm regions, and are detectable at distances up to 1 kpc by Gemini/GPI, VLT/SPHERE, and Subaru/HiCIAO/SCExAO. By contrast, collapsed clumps formed by disk fragmentation have such strong local gravitational fields that their scattering photospheres are at lower altitudes, such fragments appear fainter than their surroundings in the NIR. Spiral arms and streamers recently imaged in four FU Ori systems at NIR wavelengths resemble GI-induced structures and support the interpretation that FUors are gravitationally unstable protostellar disks. At mm wavelengths, both spirals and clumps appear brighter in thermal emission than the ambient disk and can be detected by ALMA at distances up to 0.4 kpc with one-hour integration times at ~0.1 arcsec resolution. Collapsed fragments having masses >=1 MJ can be detected by ALMA within ~10 minutes.

Constraining the physical structure of the inner few 100 AU scales of deeply-embedded low-mass protostars

Constraining the physical structure of the inner few 100 AU scales of deeply-embedded low-mass protostars

Authors:

Persson et al

Abstract:

The physical structure of deeply-embedded low-mass protostars (Class 0) on scales of less than 300 AU is still poorly constrained. Determining this is crucial for understanding the physical and chemical evolution from cores to disks. In this study two models of the emission, a Gaussian disk intensity distribution and a parametrized power-law disk model, are fitted to sub-arcsecond resolution interferometric continuum observations of five Class 0 sources, including one source with a confirmed Keplerian disk. For reference, a spherically symmetric single power-law envelope is fitted to the larger scale (∼1000 AU) emission and investigated further for one of the sources on smaller scales. A thin disk model can approximate the emission and physical structure in the inner few 100 AU scales of the studied deeply-embedded low-mass protostars and paves the way for analysis of a larger sample with ALMA. While the disk radii agree with previous estimates the masses are different for some of the sources studied. Assuming a typical temperature distribution, the fractional amount of mass in the disk above 100 K varies in between 7% to 30%. Kinematic data are needed to determine the presence of any Keplerian disk. Using previous observations of p-H182O, we estimate the relative gas phase water abundances roughly an order of magnitude higher than previously inferred when both warm and cold H2 was used as reference. A spherically symmetric single power-law envelope model fails to simultaneously reproduce both the small and large scale emission.

Magnetic Fields in Early Protostellar Disk Formation

Magnetic Fields in Early Protostellar Disk Formation

Authors:

González-Casanova et al

Abstract:

We consider formation of accretion disks from a realistically turbulent molecular gas using 3D MHD simulations. In particular, we analyze the effect of the fast turbulent reconnection described by the Lazarian & Vishniac model for the removal of magnetic flux from a disk. With our numerical simulations we demonstrate how the fast reconnection enables protostellar disk formation resolving the so-called "magnetic braking catastrophe." In particular, we provide a detailed study of the dynamics of a 0.5 M⊙ protostar and the formation of its disk for up to several thousands years. We measure the evolution of the mass, angular momentum, magnetic field, and turbulence around the star. We consider effects of two processes that strongly affect the magnetic transfer of angular momentum, both of which are based on turbulent reconnection: the first, "reconnection diffusion," removes the magnetic flux from the disk; the other involves the change of the magnetic field's topology, but does not change the absolute value of the magnetic flux through the disk. We demonstrate that for the first mechanism, turbulence causes a magnetic flux transport outward from the inner disk to the ambient medium, thus decreasing the coupling of the disk to the ambient material. A similar effect is achieved through the change of the magnetic field's topology from a split monopole configuration to a dipole configuration. We explore how both mechanisms prevent the catastrophic loss of disk angular momentum and compare both above turbulent reconnection mechanisms with alternative mechanisms from the literature.

Gas Gaps in the Protoplanetary Disk around the Young Protostar HL Tauri

Gas Gaps in the Protoplanetary Disk around the Young Protostar HL Tau

Authors:

Yen et al

Abstract:

We have analyzed the HCO+ (1-0) data of the Class I-II protostar, HL Tau, obtained from the Atacama Large Millimeter/Submillimeter Array long baseline campaign. We generated the HCO+ image cube at an angular resolution of ~0.07 (~10 AU), and performed azimuthal averaging on the image cube to enhance the signal-to-noise ratio and measure the radial profile of the HCO+ integrated intensity. Two gaps at radii of ~28 AU and ~69 AU and a central cavity are identified in the radial intensity profile. The inner HCO+ gap is coincident with the millimeter continuum gap at a radius of 32 AU. The outer HCO+ gap is located at the millimeter continuum bright ring at a radius of 69 AU and overlaps with the two millimeter continuum gaps at radii of 64 AU and 74 AU. On the contrary, the presence of the central cavity is likely due to the high optical depth of the 3 mm continuum emission and not the depletion of the HCO+ gas. We derived the HCO+ column density profile from its intensity profile. From the column density profile, the full-width-half-maximum widths of the inner and outer HCO+ gaps are both estimated to be ~14 AU, and their depths are estimated to be ~2.4 and ~5.0. These results are consistent with the expectation from the gaps opened by forming (sub-)Jovian mass planets, while placing tight constraints on the theoretical models solely incorporating the variation of dust properties and grain sizes.

The evolution of self-gravitating accretion discs

The evolution of self-gravitating accretion discs

Authors:

Rice et al

Abstract:

It is quite likely that self-gravity will play an important role in the evolution of accretion discs, in particular those around young stars, and those around supermassive black holes. We summarise, here, our current understanding of the evolution of such discs, focussing more on discs in young stellar system, than on discs in active galactic nuclei. We consider the conditions under which such discs may fragment to form bound objects, and when they might, instead, be expected to settle into a quasi-steady, self-regulated state. We also discuss how this understanding may depend on the mass of the disc relative to the mass of the central object, and how it might depend on the presence of external irradiation. Additionally, we consider whether or not fragmentation might be stochastic, where we might expect it to occur in an actual protostellar disc, and if there is any evidence for fragmentation actually playing a role in the formation of planetary-mass bodies. Although there are still a number of outstanding issue, such as the convergence of simulations of self-gravitating discs, whether or not there is more than one mode of fragmentation, and quite what role self-gravitating discs may play in the planet formation process, our general understanding of these systems seems quite robust.

Perseus Molecular Cloud's Protostars and Protostellar Disks

The VLA Nascent Disk and Multiplicity Survey: First Look at Resolved Candidate Disks around Class 0 and I Protostars in the Perseus Molecular Cloud

Authors:

Segura-Cox et al

Abstract:

We present the first dust emission results toward a sample of seven protostellar disk candidates around Class 0 and I sources in the Perseus molecular cloud from the VLA Nascent Disk and Multiplicity (VANDAM) survey with ~0.05'' or 12 AU resolution. To examine the surface brightness profiles of these sources, we fit the Ka-band 8 mm dust-continuum data in the u,v-plane to a simple, parametrized model based on the Shakura-Sunyaev disk model. The candidate disks are well-fit by a model with a disk-shaped profile and have masses consistent with known Class 0 and I disks. The inner-disk surface densities of the VANDAM candidate disks have shallower density profiles compared to disks around more evolved Class II systems. The best-fit model radii of the seven early-result candidate disks are R_c greater than 10 AU; at 8 mm, the radii reflect lower limits on the disk size since dust continuum emission is tied to grain size and large grains radially drift inwards. These relatively large disks, if confirmed kinematically, are inconsistent with theoretical models where the disk size is limited by strong magnetic braking to less than 10 AU at early times.

Protoplanetary Disk and Multiplicity Survey of Perseus Protostars

The VLA Nascent Disk and Multiplicity Survey of Perseus Protostars (VANDAM). II. Multiplicity of Protostars in the Perseus Molecular Cloud

Authors:

Tobin et al

Abstract:

We present a multiplicity study of all known protostars (94) in the Perseus molecular cloud from a Karl G. Jansky Very Large Array (VLA) survey at Ka-band (8 mm and 1 cm) and C-band (4 cm and 6.6 cm). The observed sample has a bolometric luminosity range between 0.1 L⊙ and ∼33 L⊙, with a median of 0.7 L⊙. This multiplicity study is based on the Ka-band data, having a best resolution of ∼0.065" (15 AU) and separations out to ∼43" (10000 AU) can be probed. The overall multiplicity fraction (MF) is found to be of 0.40±0.06 and the companion star fraction (CSF) is 0.71±0.06. The MF and CSF of the Class 0 protostars are 0.57±0.09 and 1.2±0.2, and the MF and CSF of Class I protostars are both 0.23±0.08. The distribution of companion separations appears bi-modal, with a peak at ∼75 AU and another peak at ∼3000 AU. Turbulent fragmentation is likely the dominant mechanism on greater than 1000 AU scales and disk fragmentation is likely to be the dominant mechanism on less than 200 AU scales. Toward three Class 0 sources we find companions separated by less than 30 AU. These systems have the smallest separations of currently known Class 0 protostellar binary systems. Moreover, these close systems are embedded within larger (50 AU to 400 AU) structures and may be candidates for ongoing disk fragmentation.

Disk Fragmentation Provides Important Mechanism for Producing low Mass Stars

The Role of Discs in the Collapse and Fragmentation of Prestellar Cores

Authors:

Lomax et al

Abstract:

Disc fragmentation provides an important mechanism for producing low mass stars in prestellar cores. Here, we describe Smoothed Particle Hydrodynamics simulations which show how populations of prestellar cores evolve into stars. We find the observed masses and multiplicities of stars can be recovered under certain conditions.

First, protostellar feedback from a star must be episodic. The continuous accretion of disc material on to a central protostar results in local temperatures which are too high for disc fragmentation. If, however, the accretion occurs in intense outbursts, separated by a downtime of ∼10^4years, gravitational instabilities can develop and the disc can fragment.

Second, a significant amount of the cores' internal kinetic energy should be in solenoidal turbulent modes. Cores with less than a third of their kinetic energy in solenoidal modes have insufficient angular momentum to form fragmenting discs. In the absence of discs, cores can fragment but results in a top heavy distribution of masses with very few low mass objects.

Protostar L1455 IRS1 Strange Disk

Protostar L1455 IRS1: Rotating Disk Connecting to Filamentary Network

Authors:

Chou et al

Abstract:

We conducted IRAM-30m C18O (2-1) and SMA 1.3mm continuum, 12CO (2-1), and C18O (2-1) observations toward the Class 0/I protostar L1455 IRS1 in Perseus. The IRAM results show L1455 IRS1 located in a dense core of 0.05 pc in size with a mass of 0.35M_sun. Besides, we identify a filamentary structure connecting to the core, exhibiting a velocity gradient of ~8.1 km s-1 pc-1 along its longitudinal axis, leading to a mass flow rate of ~1.8M_sun Myr-1. The C18O component detected in SMA shows a clear velocity gradient of ~150 km s-1 pc-1 perpendicular to the bipolar outflow seen in 12CO, and likely traces the rotational motion. Its measured rotational velocity profile is proportional to r^-0.75, and the specific angular momentum on a 500AU scale is estimated to be ~1.0x10^-3 km s-1 pc. The rotational profile becomes shallower at a turning radius of ~200 AU which is approximately the radius of the 1.3mm continuum component. These results hint the presence of a Keplerian disk with a radius less than 200au around L1455 IRS1 with a protostellar mass of about 0.28M_sun. Previous polarimetric observations show a magnetic field aligned with the outflow axis and perpendicular to the associated filament on a 0.1pc scale, while on the inner 1000 AU scale, the field becomes perpendicular to the outflow axis. The faster envelope rotation seen in L1455 IRS1 could be related to the feeding from the associated filament. The change in the magnetic field orientations is consistent with the estimated increasing rotational energy from large to small scales that overcomes the magnetic field energy, wrapping the field lines and aligning them with the disk velocity gradient. These results are discussed in the context of the interplay between filament, magnetic field, and gas kinematics from large to small scale. Possible emerging trends are explored when further comparing to a larger sample of 8 Class 0/I protostars.

Protostar IRAS 4A is Forming a Protoplanetary Disk

High Resolution 8 mm and 1 cm Polarization of IRAS 4A from the VLA Nascent Disk and Multiplicity (VANDAM) Survey

Authors:

Cox et al

Abstract:

Magnetic fields can regulate disk formation, accretion and jet launching. Until recently, it has been difficult to obtain high resolution observations of the magnetic fields of the youngest protostars in the critical region near the protostar. The VANDAM survey is observing all known protostars in the Perseus Molecular Cloud. Here we present the polarization data of IRAS 4A. We find that with ~ 0.2'' (50 AU) resolution at {\lambda} = 8.1 and 10.3 mm, the inferred magnetic field is consistent with a circular morphology, in marked contrast with the hourglass morphology seen on larger scales. This morphology is consistent with frozen-in field lines that were dragged in by rotating material entering the infall region. The field morphology is reminiscent of rotating circumstellar material near the protostar. This is the first polarization detection of a protostar at these wavelengths. We conclude from our observations that the dust emission is optically thin with {\beta} ~ 1.3, suggesting that mm/cm-sized grains have grown and survived in the short lifetime of the protostar.

V1184 Tauri had a 15 Year Brightness dip, Probably Transiting Dust Clouds

The pre-main-sequence star V1184 Tauri (CB 34V) at the end of prolonged eclipse⋆

Authors:

Semkov et al

Abstract:

Abstract

Aims.

V1184 Tau (CB 34V) lies in the field of the Bok globule CB 34 and was discovered as a large amplitude variable in 1993. According to the first hypothesis of the variability of the star, it is a FU Orionis candidate erupted between 1951 and 1993. During subsequent observations, the star manifests large amplitude variability interpreted as obscuration from circumstellar clouds of dust. We included V1184 Tau (CB 34V) in our target list of highly variable pre-main-sequence stars to determine the reasons for the variations in the brightness of this object.

Methods.

Data from BVRI photometric observations of the young stellar object V1184 Tau, obtained in the period 2008−2015, are presented in the paper. These data are a continuation of our optical photometric monitoring of the star began in 2000 and continuing to date. The photometric observations of V1184 Tau were performed in two observatories with two medium-sized and two small telescopes.

Results.

Our results indicate that during periods of maximum light the star shows characteristics typical of T Tauri stars. During the observed deep minimum in brightness, however, V1184 Tau is rather similar to UX Orionis objects. The deep drop in brightness began in 2003 ended in 2015 as the star has returned to maximum light. The light curve during the drop is obviously asymmetric as the decrease in brightness lasts two times longer than the rise. The observed colour reverse on the colour-magnitude diagrams is also confirmation of obscuration from circumstellar clouds of dust as a reason for the large amplitude variability in the brightness.

Detection of Infall in the Protostar B335

Detection of Infall in the Protostar B335 with ALMA

Authors:

Evans et al

Abstract:

Observations of the isolated globule B335 with ALMA have yielded absorption features against the continuum that are redshifted from the systemic velocity in both HCN and HCO+ lines. These features provide unambiguous evidence for infall toward a central luminosity source. Previously developed models of inside-out collapse can match the observed line profiles of HCN and HCO+ averaged over the central 50 AU. At the new distance of 100 pc, the inferred infall radius is 0.012 pc, the mass infall rate is 3×10−6 solar masses per year, the age is 50,000 years, and the accumulated mass in the central zone is 0.15 solar masses, most of which must be in the star or in parts of a disk that are opaque at 0.8 mm. The continuum detection indicates an optically thin mass (gas and dust) of only 7.5×10−4 solar masses in the central region, consistent with only a very small disk mass.

No Keplerian Disk >10 AU around the Protostar B335: Magnetic Braking or Young Age?

No Keplerian Disk greater than 10 AU around the Protostar B335: Magnetic Braking or Young Age?

Authors:

Yen et al

Abstract:

We have conducted ALMA cycle 2 observations in the 1.3 mm continuum and in the C18O (2-1) and SO (5_6-4_5) lines at a resolution of ~0.3" toward the Class 0 protostar B335. The 1.3 mm continuum, C18O, and SO emission all show central compact components with sizes of ~40-180 AU within more extended components. The C18O component shows signs of infalling and rotational motion. By fitting simple kinematic models to the C18O data, the protostellar mass is estimated to be 0.05 Msun. The specific angular momentum, on a 100 AU scale, is ~4.3E-5 km/s*pc. A similar specific angular momentum, ~3E-5 to 5E-5 km/s*pc, is measured on a 10 AU scale from the velocity gradient observed in the central SO component, and there is no clear sign of an infalling motion in the SO emission. By comparing the infalling and rotational motion, our ALMA results suggest that the observed rotational motion has not yet reached Keplerian velocity neither on a 100 AU nor even on a 10 AU scale. Consequently, the radius of the Keplerian disk in B335 (if present) is expected to be 1-3 AU. The expected disk radius in B335 is one to two orders of magnitude smaller than those of observed Keplerian disks around other Class 0 protostars. Based on the observed infalling and rotational motion from 0.1 pc to inner 100 AU scales, there are two possible scenarios to explain the presence of such a small Keplerian disk in B335: magnetic braking and young age. If our finding is the consequence of magnetic braking, ~50% of the angular momentum of the infalling material within a 1000 AU scale might have been removed, and the magnetic field strength on a 1000 AU scale is estimated to be ~200 uG. If it is young age, the infalling radius in B335 is estimated to be ~2700 AU, corresponding to a collapsing time scale of ~5E4 yr.