Star Formation Research Papers - Academia.edu (original) (raw)

Star clusters can be found in galaxy mergers, not only in central regions, but also in the tidal debris. In both the Eastern and Western tidal tails of NGC 3256 there are dozens of young star clusters, confirmed by their blue colors and... more

Star clusters can be found in galaxy mergers, not only in central regions, but also in the tidal debris. In both the Eastern and Western tidal tails of NGC 3256 there are dozens of young star clusters, confirmed by their blue colors and larger concentration index as compared to sources off of the tail. Tidal tails of other galaxy pairs do not have such widespread cluster formation, indicating environmental influences on the process of star formation or the packaging of the stars.

We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even... more

We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even though the feedback is negative, the solutions can exhibit cyclic and episodic solutions. We find that periodic or episodic star formation only occurs when two conditions

In the infrared, the heavily reddened LkH$\alpha$ 101 is one of the brightest young stars in the sky. Situated just north of the Taurus-Auriga complex in the L1482 dark cloud, it appears to be an early B-type star that has been... more

In the infrared, the heavily reddened LkH$\alpha$ 101 is one of the brightest young stars in the sky. Situated just north of the Taurus-Auriga complex in the L1482 dark cloud, it appears to be an early B-type star that has been serendipitously exposed during a rarely observed stage of early evolution, revealing a remarkable spectrum and a directly-imaged circumstellar disk. While detailed studies of this star and its circumstellar environment have become increasingly sophisticated in the 50 years since Herbig (1956) first pointed it out, the true nature of the object still remains a mystery. Recent work has renewed focus on the young cluster of stars surrounding LkH$\alpha$ 101, and what it can tell us about the enigmatic source at its center (e.g., massive star formation timescales, clustered formation mechanisms). This latter effort certainly deserves more intensive study. We describe the current knowledge of this region and point out interesting work that could be done in the future.

My work here is the Determine Age of Open Star Clusters. Star clusters, as the name suggests are very large group of stars that are gravitationally bound. They are at the same distance from earth and are of the same age. By analyzing them... more

My work here is the Determine Age of Open Star Clusters. Star clusters, as the name suggests are very large group of stars that are gravitationally bound. They are at the same distance from earth and are of the same age. By analyzing them we can find information about the evolution of different type of stars and clusters. There can be two types of clusters; Open star clusters and Globular star clusters. On comparison with Globular star clusters, Open star clusters are gravitationally loosely bound and young, mainly consisting of blue stars. Open clusters will often disrupt due to the gravitational influence of nearby giant molecular cloud even before the stars in it die. It will be having a few hundreds of stars and they are confined to the galactic planes, in the spiral arms of the galaxies. In this project I am using the data from Gaia DR2, which comprises of 46 open star clusters. I find the age of these clusters using two methods; Using theory of stellar evolution and Stellar isochrone fitting methods with the help of Excel and TOPCAT software.

A Monte Carlo computer model of extra-solar planetary formation and evolution, which includes the planetary geochemical carbon cycle, is presented. The results of a run of one million galactic disc stars are shown where the aim was to... more

A Monte Carlo computer model of extra-solar planetary formation and evolution, which includes the planetary geochemical carbon cycle, is presented. The results of a run of one million galactic disc stars are shown where the aim was to assess the possible abundance of both biocompatible and habitable planets. (Biocompatible planets are defined as worlds where the long-term presence of surface liquid water provides environmental conditions suitable for the origin and evolution of life. Habitable planets are those worlds with more specifically Earth-like conditions.) The model gives an estimate of 1 biocompatible planet per 39 stars, with the subset of habitable planets being much rarer at 1 such planet per 413 stars. The nearest biocompatible planet may thus lie ~ 14 LY distant and the nearest habitable planet ~ 31 LY away. If planets form in multiple star systems then the above planet/star ratios may be more than doubled. By applying these results to stars in the solar neighbourhood, it is possible to identify 28 stars at distances of < 22 LY with a non-zero probability of possessing a biocompatible planet.

""Knowledge of the physical and chemical processes which occur within the circum-stellar nebula of a newly formed star is still uncertain and permits a number of distinct theories to account for the formation of planets. In order to gain... more

""Knowledge of the physical and chemical processes which occur within the circum-stellar nebula of a newly formed star is still uncertain and permits a number of distinct theories to account for the formation of planets. In order to gain a greater insight into the complexities of planetary formation, researchers have concentrated on 'realistic' simulations of clearly defined and limited aspects of the problem of the formation of the Solar System. Attempts to describe the nature of planetary systems of other stars have been left largely to popular speculation.
The microcomputer model presented here produces a wide range of data for possible planetary systems with primary stars in the mass range 0.6 - 1.3 M⊙. A synthesis of current theory, research and speculation, the purpose of this model is not to add to our understanding of the processes that form planets, but to give an integrated view of the possible nature of extra-solar planetary systems and to investigate the possibility of a systematic variation in planetary characteristics with primary mass. ""

The 3.3 micron PAH feature is undetected for the majority of the sample (97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra, for a source... more

The 3.3 micron PAH feature is undetected for the majority of the sample (97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra, for a source that is borderline embedded. For all 12 sources with both VLT and Spitzer spectra, no PAH features are detected in either. In total, PAH features are detected toward at most 1 out of 63 (candidate) embedded protostars (<~ 2%), even lower than observed for class II T Tauri stars with disks (11-14%). Assuming typical class I stellar and envelope parameters, the absence of PAHs emission is most likely explained by the absence of emitting carriers through a PAH abundance at least an order of magnitude lower than in molecular clouds but similar to that found in disks. Thus, most PAHs likely enter the protoplanetary disks frozen out in icy layers on dust grains and/or in coagulated form.

Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation:... more

Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation: The physics of the ISM. Stars form from dense molecular clouds that contain ∼ 30% of the total interstellar medium (ISM) mass. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system – the ISM. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. Most of the ISM volume (∼ 65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. The physics of accretion and outflow. Powerful outflows are known to regulate angular momentum transport during star formation, the so-called accretion–outflow engine. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1 AU) where the gravitational field is strong. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1 AU) from its parent star. The temperature of the accretion–outflow engine is between 3000 and 10 7 K. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The physics of planet formation. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. This region is illuminated by the strong UV radiation field produced by the star and the accretion–outflow engine. Absorption spectroscopy provides the most sensitive tool for determining the properties of the circumstellar gas as well as the characteristics of the atmospheres of the inner planets transiting the stellar disk. UV radiation also pumps the electronic transitions of the most abundant molecules (H 2, CO, etc.) that are observed in the UV. Here we argue that access to the UV spectral range is essential for making progress in this field, since the resonance lines of the most abundant atoms and ions at temperatures between 3000 and 300 000 K, together with the electronic transitions of the most abundant molecules (H 2, CO, OH, CS, S 2, CO 2+, C 2, O 2, O3, etc.) are at UV wavelengths. A powerful UV-optical instrument would provide an efficient mean for measuring the abundance of ozone in the atmosphere of the thousands of transiting planets expected to be detected by the next space missions (GAIA, Corot, Kepler, etc.). Thus, a follow-up UV mission would be optimal for identifying Earth-like candidates.

We describe the Galaxy Evolution Explorer (GALEX) satellite that was launched in April 2003 specifically to accomplish far ultraviolet (FUV) and near ultraviolet (NUV) imaging and spectroscopic sky-surveys. GALEX is currently providing... more

We describe the Galaxy Evolution Explorer (GALEX) satellite that was launched in April 2003 specifically to accomplish far ultraviolet (FUV) and near ultraviolet (NUV) imaging and spectroscopic sky-surveys. GALEX is currently providing new and significant information on how ...

Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connec- tions. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma... more

Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connec- tions. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma associated with a given magnetic field line at one instant is distributed along a different set of magnetic field lines at the next instant. This diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed ”reconnec- tion diffusion”. The astrophysical implications of this con- cept include heat transfer in plasmas, advection of heavy el- ements in interstellar medium, magnetic field generation etc. However, the most dramatic implications of the concept are related to the star formation process. The reason is that mag- netic fields are dynamically important for most of the stages of star formation. The existing theory of star formation has been developed ignoring the possibility of reconnection dif- fusion. Instead, it appeals to the decoupling of mass and magnetic field arising from neutrals drifting in respect to ions entrained on magnetic field lines, i.e. through the pro- cess that is termed ”ambipolar diffusion”. The predictions of ambipolar diffusion and reconnection diffusion are very different. For instance, if the ionization of media is high, am- bipolar diffusion predicts that the coupling of mass and mag- netic field is nearly perfect. At the same time, reconnection diffusion is independent of the ionization but depends on the scale of the turbulent eddies and on the turbulent velocities. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view, i.e. appealing to the reconnection of flux tubes and to the dif-
the dynamics of many key processes, including magnetic re- connection. Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connec- tions. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma associated with a given magnetic field line at one instant is distributed along a different set of magnetic field lines at the next instant. This diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed ”reconnec- tion diffusion”. The astrophysical implications of this con- cept include heat transfer in plasmas, advection of heavy el- ements in interstellar medium, magnetic field generation etc. However, the most dramatic implications of the concept are related to the star formation process. The reason is that mag- netic fields are dynamically important for most of the stages of star formation. The existing theory of star formation has been developed ignoring the possibility of reconnection dif- fusion. Instead, it appeals to the decoupling of mass and magnetic field arising from neutrals drifting in respect to ions entrained on magnetic field lines, i.e. through the pro- cess that is termed ”ambipolar diffusion”. The predictions of ambipolar diffusion and reconnection diffusion are very different. For instance, if the ionization of media is high, am- bipolar diffusion predicts that the coupling of mass and mag- netic field is nearly perfect. At the same time, reconnection diffusion is independent of the ionization but depends on the scale of the turbulent eddies and on the turbulent velocities. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view. We make use of the Lazarian & Vishniac 1999 theory of magnetic reconnection and show that this theory is applicable to the partially ionized gas. We quantify the reconnection diffusion rate both for weak and strong MHD turbulence and address the problem of recon- nection diffusion acting together with ambipolar diffusion. In addition, we provide a criterion for correctly representing the magnetic diffusivity in simulations of star formation. We discuss the intimate relation between the processes of recon- nection diffusion, field wandering and turbulent mixing of a magnetized media and show that the role of the plasma ef- fects is limited to ”breaking up lines” on small scales and does not affect the rate of reconnection diffusion. We ad- dress the existing observational results and demonstrate how reconnection diffusion can explain the puzzles presented by observations, in particular, the observed higher magnetiza- tion of cloud cores in comparison with the magnetization of envelopes. We also outline a possible set of observational tests of the reconnection diffusion concept and discuss how the application of the new concept changes our understand- ing of star formation and its numerical modeling. Finally, we outline the differences of the process of reconnection diffusion and the process of accumulation of matter along magnetic field lines that is frequently invoked to explain the results of numerical simulations

We report Mopra Australia Telescope National Facility (ATNF), Anglo-Australian Telescope and Atacama Submillimeter Telescope Experiment observations of a molecular clump in Carina, BYF73 = G286.21+0.17, which give evidence of large-scale... more

We report Mopra Australia Telescope National Facility (ATNF), Anglo-Australian Telescope and Atacama Submillimeter Telescope Experiment observations of a molecular clump in Carina, BYF73 = G286.21+0.17, which give evidence of large-scale gravitational infall in the dense gas. From the millimetre and far-infrared data, the clump has a mass of ~2 × 104Msolar, luminosity of ~2-3 × 104Lsolar and diameter of ~0.9 pc. From radiative transfer modelling, we derive a mass infall rate of ~3.4 × 10-2Msolaryr-1. If confirmed, this rate for gravitational infall in a molecular core or clump may be the highest yet seen. The near-infrared K-band imaging shows an adjacent compact HII region and IR cluster surrounded by a shell-like photodissociation region showing H2 emission. At the molecular infall peak, the K imaging also reveals a deeply embedded group of stars with associated H2 emission. The combination of these features is very unusual, and we suggest that they indicate the ongoing formation ...

Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls in the center of the cloud, creating a protoplanetary disk surrounding a protostar. Planets and other solar system bodies will be formed in... more

Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls in the center of the cloud, creating a protoplanetary disk surrounding a protostar. Planets and other solar system bodies will be formed in the disk.The chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these objects.I studied the chemical and physical evolution of this matter, from the cloud to the disk, using the chemical gas-grain code Nautilus.A sensitivity study to some parameters of the code (such as elemental abundances and parameters of grain surface chemistry) has been done. More particularly, the updates of rate coefficients and branching ratios of the reactions of our chemical network showed their importance, such as on the abundances of some chemical species, and on the code sensitivity to others parameters.Several physical models of collapsing dense core have also been considered. The more complex and solid approach has been to interface our chemical code with the radiation-magneto-hydrodynamic model of stellar formation RAMSES, in order to model in three dimensions the physical and chemical evolution of a young disk formation. Our study showed that the disk keeps imprints of the past history of the matter, and so its chemical composition is sensitive to the initial conditions.

It is conjectured that phenomena in biological microworld could be equated with astrophysical principles of gravitation. Fluids, gases that constitute 50-90 % of the total accredited biomass causes seclusion from stronger external... more

It is conjectured that phenomena in biological microworld could be equated with astrophysical principles of gravitation. Fluids, gases that constitute 50-90 % of the total accredited biomass causes seclusion from stronger external gravitational fields. Seclusion due to buoyant condition is reflected in apparent ‘weight’ reduced. The g-value (self) to the tune of nanometer per second square in a massive body of a planet may be negligible, but in an isolated living mass at picometer distance, such acceleration is quite a significant force. Homogeneous and heterogeneous accretion; heating and cooling phenomena as well as the rhythmic pattern of growth due to elastic collisions in massive star; heat transfer mechanisms viz. radiation, (perspiration), conduction and convection; site of human core body temperature at liver, kidney, heart and parts of brain at central region under initial circular fetus situation; or coldest part of the periphery at toe, foot, hand demonstrates similarity between biological and astrophysical words. Mechanical movement of macromolecules under neutral buoyant condition of the principle ‘heavier molar mass plus higher the density- faster the central travel from periphery’ for nucleic acid and protein; fats & lipids in addition to molar mass and density- variation in temperature; distribution carbohydrates in protoplasm under miscible condition due to solubility under granular soil and gel properties were equated with movement phenomena under astrophysical principle. Centrifugation as standard protocol for separating organelles is considered as inversion or reversing the natural setting on losing its compression memory.

How mass is accumulated from cloud-scale down to individual stars is a key open question in understanding high-mass star formation. Here, we present the mass accumulation process in a hub-filament cloud G22 that is composed of four... more

How mass is accumulated from cloud-scale down to individual stars is a key open question in understanding high-mass star formation. Here, we present the mass accumulation process in a hub-filament cloud G22 that is composed of four supercritical filaments. Velocity gradients detected along three filaments indicate that they are collapsing with a total mass infall rate of about 440 M e Myr −1 , suggesting the hub mass would be doubled in six free-fall times, adding up to ∼2 Myr. A fraction of the masses in the central clumps C1 and C2 can be accounted for through large-scale filamentary collapse. Ubiquitous blue profiles in HCO + (3–2) and 13 CO(3–2) spectra suggest a clump-scale collapse scenario in the most massive and densest clump C1. The estimated infall velocity and mass infall rate are 0.31 km s −1 and 7.2×10 −4 M e yr −1 , respectively. In clump C1, a hot molecular core (SMA1) is revealed by the Submillimeter Array observations and an outflow-driving high-mass protostar is located at the center of SMA1. The mass of the protostar is estimated to be 11–15 M e and it is still growing with an accretion rate of 7×10 −5 M e yr −1. The coexistent infall in filaments, clump C1, and the central hot core in G22 suggests that pre-assembled mass reservoirs (i.e., high-mass starless cores) may not be required to form high-mass stars. In the course of high-mass star formation, the central protostar, the core, and the clump can simultaneously grow in mass via core-fed/disk accretion, clump-fed accretion, and filamentary/cloud collapse.

We present multi-wavelengths observations and a radiative transfer model of a newly discovered massive circumstellar disk of gas and dust which is one of the largest disks known today. Seen almost edge-on, the disk is resolved in... more

We present multi-wavelengths observations and a radiative transfer model of a newly discovered massive circumstellar disk of gas and dust which is one of the largest disks known today. Seen almost edge-on, the disk is resolved in high-resolution near-infrared (NIR) images and appears as a dark lane of high opacity intersecting a bipolar reflection nebula. Based on molecular line observations we estimate the distance to the object to be 3.5 kpc. This leads to a size for the dark lane of ~10500 AU but due to shadowing effects the true disk size could be smaller. In Spitzer/IRAC 3.6 micron images the elongated shape of the bipolar reflection nebula is still preserved and the bulk of the flux seems to come from disk regions that can be detected due to the slight inclination of the disk. At longer IRAC wavelengths, the flux is mainly coming from the central regions penetrating directly through the dust lane. Interferometric observations of the dust continuum emission at millimeter wavelengths with the SMA confirm this finding as the peak of the unresolved mm-emission coincides perfectly with the peak of the Spitzer/IRAC 5.8 micron flux and the center of the dark lane seen in the NIR images. Simultaneously acquired CO data reveal a molecular outflow along the northern part of the reflection nebula which seems to be the outflow cavity. An elongated gaseous disk component is also detected and shows signs of rotation. The emission is perpendicular to the molecular outflow and thus parallel to but even more extended than the dark lane in the NIR images. Based on the dust continuum and the CO observations we estimate a disk mass of up to a few solar masses depending on the underlying assumptions. Whether the disk-like structure is an actual accretion disk or rather a larger-scale flattened envelope or pseudodisk is difficult to discriminate with the current dataset (abridged).

We have analyzed Spitzer and NASA/IRTF 2 - 35 \mum spectra of the warm, ~350 K circumstellar dust around the nearby MS star {\eta} Corvi (F2V, 1.4 \pm 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ~3... more

We have analyzed Spitzer and NASA/IRTF 2 - 35 \mum spectra of the warm, ~350 K circumstellar dust around the nearby MS star {\eta} Corvi (F2V, 1.4 \pm 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ~3 AU from the central star, in the system's Terrestrial Habitability Zone. Spectral features due to ultra-primitive cometary material were found, in addition to features due to impact produced silica and high temperature carbonaceous phases. At least 9 x 10^18 kg of 0.1 - 100 \mum warm dust is present in a collisional equilibrium distribution with dn/da ~ a^-3.5, the equivalent of a 130 km radius KBO of 1.0 g/cm^3 density and similar to recent estimates of the mass delivered to the Earth at 0.6 - 0.8 Gyr during the Late Heavy Bombardment. We conclude that the parent body was a Kuiper-Belt body or bodies which captured a large amount of early primitive material in the first Myrs of the system's lifetime and preserved it in deep freeze at ~150 AU. At ~1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10 km/sec with a rocky planetary body of mass \leq MEarth at ~3 AU, delivering large amounts of water (>0.1% of MEarth's Oceans) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO.

Water is a key molecule for determining the physical and chemical structure of star-forming regions because of its large abundance variations, both in the gas and in the ice, between warm and cold regions. In this HIFI-led 429 hr Key... more

Water is a key molecule for determining the physical and chemical structure of star-forming regions because of its large abundance variations, both in the gas and in the ice, between warm and cold regions. In this HIFI-led 429 hr Key Program, we are obtaining a comprehensive set of water observations toward a large sample of well-characterized protostars, covering a wide range of masses and luminosities -from the lowest to the highest mass protostars-, as well as evolutionary stages -from the first stages represented by pre-stellar cores to the later stages represented by the pre-main sequence stars surrounded only by their protoplanetary disks. Lines of H2O and its isotopologues, as well as chemically related hydrides, are observed. In addition, selected high-frequency lines of CO isotopes, [O I] and [C II] are obtained with HIFI and PACS, and are complemented by ground-based HDO, CO and dust continuum maps to ensure a self-consistent data set for analysis. Limited mapping information on a few arcmin scale provides information on local variations due to outflows and clustered star formation. Together, the data elucidate the physical processes responsible for the warm gas (passive heating, UV or X-ray-heating, shocks, disks), probe dynamical processes associated with forming stars and planets (outflow, infall, turbulence), reveal the chemical evolution of water and the oxygen-reservoir, and test basic gas-grain chemical interactions. By the time of the COSPAR meeting, about half of our program should have been observed. This talk will present an overview of the main highlights.