arXiv subjects

Daniel Harsono

Publications and source records attributed to Daniel Harsono.

64 records · Page 4Linked to original sources

arXiv2014

ALMA hints at the presence of two companions in the disk around HD 100546

HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present ALMA observations of the HD 100546 disk which resolve the gas and dust structure at (sub)mm wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390+/-20 AU in radius) whereas the continuum emission is more compact (230+/-20 AU in radius) suggesting radial drift of the mm-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and um-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75+/-3 AU centered at 190+/-3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of mm-sized grains within a compact ring, ~ 20 - 30 AU, by ~ 10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (~ 1.2 Myr) and also triggers the development of an outer ring (~ 100 - 200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk.

astro-ph.EP↗

arXiv2014

ALMA observations of the kinematics and chemistry of disc formation

Context: The R CrA cloud hosts a handful of Class 0/I low-mass young stellar objects. The chemistry and physics at scales $>500$ AU in this cloud are dominated by the irradiation from the nearby Herbig Be star R CrA. The luminous large-scale emission makes it necessary to use high-resolution spectral imaging to study the chemistry and dynamics of the inner envelopes and discs of the protostars. Aims: We aim to better understand the structure of the inner regions of these protostars and, in particular, the interplay between the chemistry and the presence of discs. Methods: Using Atacama Large Millimeter/submillimeter Array (ALMA) high-resolution spectral imaging interferometry observations, we study the molecular line and dust continuum emission at submillimetre wavelengths. Results: We detect dust continuum emission from four circumstellar discs around Class 0/I objects within the R CrA cloud. Towards IRS7B we detect C$^{17}$O emission showing a rotation curve consistent with a Keplerian disc with a well-defined edge that gives a good estimate for the disc radius at 50 AU. We derive the central object mass to $2.3M_{\odot}$ and the disc mass to $0.024M_{\odot}$. The observations are also consistent with a model of material infalling under conservation of angular momentum; however, this model provides a worse fit to the data. We also report a likely detection of faint CH$_3$OH emission towards this point source, as well as more luminous CH$_3$OH emission in an outflow orthogonal to the major axis of the C$^{17}$O emission. Conclusions: The faint CH$_3$OH emission seen towards IRS7B can be explained by a flat density profile of the inner envelope caused by the disc with a radius $\lesssim50$ AU. We propose that the regions of the envelopes where complex organic molecules are present in Class 0/I young stellar objects can become quenched as the disc grows.

astro-ph.SR↗

arXiv2013

Rotationally-supported disks around Class I sources in Taurus: disk formation constraints

(Abridged) Disks are observed around pre-main sequence stars, but how and when they form is still heavily debated. While disks around young stellar objects have been identified through thermal dust emission, spatially and spectrally resolved molecular line observations are needed to determine their nature. We present subarcsecond observations of dust and gas toward four Class I low-mass young stellar objects in Taurus. The 13CO and C18O J=2-1 transitions at 220 GHz were observed with the Plateau de Bure Interferometer at a spatial resolution of ~0.8'' and analyzed using uv-space position velocity diagrams to determine the nature of their observed velocity radient. Rotationally supported disks (RSDs) are detected around 3 of the 4 Class I sources studied. The derived masses identify them as Stage I objects; i.e., their stellar mass is higher than their envelope and disk masses. The outer radii of the Keplerian disks toward our sample of Class I sources are <~ 100 AU. The lack of on-source C18O emission for TMR1 puts an upper limit of 50 AU on its size. Flattened structures at radii > 100 AU around these sources are dominated by infalling motion (v propto r^-1). A large-scale envelope model is required to estimate the basic parameters of the flattened structure from spatially resolved continuum data. Similarities and differences between the gas and dust disk are discussed. Combined with literature data, the sizes of the RSDs around Class I objects are best described with evolutionary models with an initial rotation of 10^-14 Hz and slow sound speeds. Based on the comparison of gas and dust disk masses, little CO is frozen out within 100 AU in these disks. RSDs with radii up to 100 AU are present around Class I embedded objects. Larger surveys of both Class 0 and I objects are needed to determine whether most disks form late or early in the embedded phase.

astro-ph.GA↗

arXiv2013

A recent accretion burst in the low-mass protostar IRAS 15398-3359: ALMA imaging of its related chemistry

Low-mass protostars have been suggested to show highly variable accretion rates through-out their evolution. Such changes in accretion, and related heating of their ambient envelopes, may trigger significant chemical variations on different spatial scales and from source-to-source. We present images of emission from C17O, H13CO+, CH3OH, C34S and C2H toward the low-mass protostar IRAS 15398-3359 on 0.5" (75 AU diameter) scales with the Atacama Large Millimeter/submillimeter Array (ALMA) at 340 GHz. The resolved images show that the emission from H13CO+ is only present in a ring-like structure with a radius of about 1-1.5" (150-200 AU) whereas the CO and other high dipole moment molecules are centrally condensed toward the location of the central protostar. We propose that HCO+ is destroyed by water vapor present on small scales. The origin of this water vapor is likely an accretion burst during the last 100-1000 years increasing the luminosity of IRAS 15398-3359 by a factor of 100 above its current luminosity. Such a burst in luminosity can also explain the centrally condensed CH3OH and extended warm carbon-chain chemistry observed in this source and furthermore be reflected in the relative faintness of its compact continuum emission compared to other protostars.

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

High-J CO survey of low-mass protostars observed with Herschel-HIFI

The evolution of deeply embedded young stellar objects (YSOs) has traditionally been traced through dust continuum spectral energy distributions (SEDs), but the use of CO excitation as an evolutionary probe has not yet been explored due to lack of high-J CO observations. The aim is to constrain the physical characteristics (excitation, kinematics, column density) of the warm gas toward 26 low-mass Class 0 and I YSOs using spectrally-resolved Herschel Space Observatory data of high-J lines of CO. Data are complemented by ground-based observations from APEX and the JCMT to compare those with the colder gas traced by lower-J CO lines. This is the first large spectrally resolved high-J CO survey conducted for these types of sources. The median excitation temperatures for 12CO, 13CO and C18O derived from single-temperature fits to the J_u=2-10 integrated intensities are ~70K, 48K and 37K, respectively, with no significant difference between Class 0 and I sources and no trend with M_env or L_bol. Thus, in contrast with the continuum SEDs, the spectral line energy distributions (SLEDs) do not show an evolution during the embedded stage. In contrast, the integrated line intensities of all CO isotopologs show a clear decrease with evolutionary stage as the envelope is dispersed. The H2O 110-101/CO 10-9 intensity ratio does not change significantly with velocity, in contrast with the H2O/CO 3-2 ratio, indicating that CO 10-9 is the lowest transition for which the line wings probe the same warm shocked gas as H2O. Modeling of the full suite of C18O lines indicates an abundance profile for Class 0 sources that is consistent with a freeze-out zone below 25 K and evaporation at higher temperatures, with but some fraction of the CO transformed into other species in the cold phase. In contrast, observations for two Class I sources in Ophiuchus are consistent with a constant high CO abundance profile.

astro-ph.SR↗

arXiv2013

Evolution of CO lines in time-dependent models of protostellar disk formation

(Abridged) Star and planet formation theories predict an evolution in the density, temperature, and velocity structure as the envelope collapses and forms an accretion disk. The aim of this work is to model the evolution of the molecular excitation, line profiles, and related observables during low-mass star formation. Specifically, the signatures of disks during the deeply embedded stage are investigated. Semi-analytic 2D axisymmetric models have been used to describe the evolution of the density, stellar mass, and luminosity from the pre-stellar to the T-Tauri phase. A full radiative transfer calculation is carried out to accurately determine the time-dependent dust temperatures and CO abundance structure. We present non-LTE near-IR, FIR, and submm lines of CO have been simulated at a number of time steps. In contrast to the dust temperature, the CO excitation temperature derived from submm/FIR lines does not vary during the protostellar evolution, consistent with C18O observations obtained with Herschel and from ground-based telescopes. The near-IR spectra provide complementary information to the submm lines by probing not only the cold outer envelope but also the warm inner region. The near-IR high-J (>8) absorption lines are particularly sensitive to the physical structure of the inner few AU, which does show evolution. High signal-to-noise ratio subarcsec resolution data with ALMA are needed to detect the presence of small rotationally supported disks during the Stage 0 phase and various diagnostics are discussed.

astro-ph.SR↗

arXiv2009

The evolution of cluster dwarfs

We summarize the results from analyzing six clusters of galaxies at 0.14 < z < 0.40 observed with the Hubble Space Telescope Advanced Camera for Surveys. We derive deep composite luminosity functions in B,g,V,r,i and z down to an absolute magnitude of -14 +5 log h mag. The luminosity functions are fitted by a single Schechter function with M^*=-19.8,-20.9,-21.9, -22.0,-21.7 and -22.3 i B,g,V.r,i,z respectively and alpha=-1.3 for all bands. The data suggest that the red sequence dominates the luminosity function down to more than 6 mag. below L*, the dwarf spheroidal regime. Hence, at least at z=0.3 the red sequence is well established and galaxies down to dwarf spheroidals are fully assembled within these clusters. We do not detect the faint-end upturn (M > -16) that is observed in lower redshift clusters. If this is real, the faint-end population has originated since z = 0.3.

astro-ph.CO↗

arXiv2008

The luminosity function of galaxies to $M_{BgVriz} \sim -14$ in $z \sim 0.3$ clusters

We present deep composite luminosity functions in $B$, $g$, $V$, $r$, $i$ and $z$ for six clusters at $0.14 < z < 0.40$ observed with the Hubble Space Telescope Advanced Camera for Surveys. The luminosity functions reach to absolute magnitude of $\sim -14 + 5\log h$ mag. and are well fitted by a single Schechter function with $M^*_{BgVriz}=-19.8, -20.9 -21.9, -22.0, -21.7, -22.3$ mag. and $α\sim -1.3$ (in all bands). The observations suggest that the galaxy luminosity function is dominated by objects on the red sequence to at least 6 mags. below the $L^*$ point. Comparison with local data shows that the red sequence is well established at least at $z \sim 0.3$ down to $\sim 1/600^{th}$ of the luminosity of the Milky Way and that galaxies down to the regime of dwarf spheroidals have been completely assembled in clusters at this redshift. We do not detect a steepening of the luminosity function at $M > -16$ as is observed locally. If the faint end upturn is real, the steepening of the luminosity function must be due to a newly infalling population of faint dwarf galaxies.

astro-ph↗

arXiv2007

The cluster galaxy luminosity function at $z=0.3$: a recent origin for the faint-end upturn ?

We derive deep luminosity functions (to $M_z=-15$) for galaxies in Abell 1835 ($z=0.25$) and AC 114 ($z=0.31$) and compare these with the local $z'$ luminosity function for 69 clusters. The data show that the faint-end upturn, the excess of galaxies above a single Schechter function at $M_z < -17$, does not exist in the higher redshift clusters. This suggests that the faint-end upturn galaxies have been created recently, by infall into clusters of star-forming field populations or via tidal disruption of brighter objects.^M

astro-ph↗