Chandra's First Decade of Discovery

Stars and Star Formation

The Cycles of Alpha Centauri

Thomas Ayres, University of Colorado (CASA)

The main AB pair of the nearby Alpha Centauri triple system has an extensive X-ray history, covering three decades. Alpha Cen A (G2V) is a near twin of the Sun, with a similarly soft (1-2 MK) corona. Alpha Cen B (K1V) is more active than the Sun, with a generally harder X-ray spectrum. Here, spatially resolved measurements from ROSAT, XMM-Newton, and Chandra are compared on a common basis. In the combined time series, Alpha Cen B shows a distinct X-ray modulation with a period of about eight years and a factor of 5 cycle depth (the latter is similar to the Sun's). Alpha Cen A showed minimal variability 1995-2000, a decrease in the initial XMM epoch 2003-2005, and nearly constant behavior in the subsequent Chandra segment (late-2005 to present), although down a factor of 2 from the mid-1990's level. A remarkable “smoking gun&8221; LETGS spectrum in mid-2007 emphasized that much of the coronal luminosity of solar-activity objects falls at longer wavelengths than recorded efficiently by contemporary instruments. This makes cycle depth strongly dependent on the energy bandpass of the measurement (here 0.2-2 keV), and complicates assessments of coronal heating requirements.

Multi-wavelength diagnostics of accretion in an X-ray selected sample of CTTSs

Rachel Curran, INAF - Osservatorio Astronomico di Palermo
Costanza Argiroffi (INAF - Osservatorio Astronomico di Palermo), Giuseppe Germano Sacco (Rochester Institute for Technology, INAF - Osservatorio Astronomico di Palermo), Salvatore Orlando (INAF - Osservatorio Astronomico di Palermo), Fabio Reale (INAF - Osservatorio Astronomico di Palermo), Giovanni Peres (INAF - Osservatorio Astronomico di Palermo), Antonio Maggio (INAF - Osservatorio Astronomico di Palermo)

The majority of CTTSs observed to date with high spectral resolution X-ray spectroscopy reveal soft X-ray emission (E<0.7 KeV) which originates from cool (1-5 MK), high density (n ~ 1011-1013 cm-3) plasma. This is currently interpreted to be due to mass accretion. Supporting this interpretation is the fact that this plasma component is too dense to have a coronal origin, and it has never been observed in non-accreting stars. Synthesized X-ray spectra from detailed hydrodynamical modelling of the interaction between the accretion flow and the stellar chromosphere also confirm this interpretation. However, the mass accretion rates derived from X-ray data are consistently underestimated when compared to mass accretion rates derived from UV/optical data. We test the hypothesis that this soft X-ray emission originates from accretion by analysing optical, NIR and X-ray data for an X-ray selected sample of CTTSs. We derive mass accretion rates for the sample based on Hα, Hei, Oi and Caii emission lines, along with the X-ray data. We draw comparisons between these mass accretion rates to understand the underestimation of the X-ray derived mass accretion rates. We discuss the possibilities of a) the X-ray emission being partially absorbed, b) the optical/NIR emission arising from different parts of the accretion stream and c) the uncertainties involved in the estimation of the mass accretion rates from different spectroscopic diagnostics.

The Chandra Cygnus OB2 Survey

Jeremy Drake, Smithsonian Astrophysical Observatory
Nicholas Wright (SAO) and the Chandra Cyg OB2 Team

Understanding massive “starburst” clusters is crucial for the first stars, starburst galaxies, galactic nucleosynthesis, evolution and ISM mixing, and star and planet formation on universal and Galactic scales. But our few known Galactic superclusters are generally too distant for detailed study. Recent awareness that the ~2 Myr old Cygnus OB2 association, at only 1.5kpc, has supercluster characteristics, has prompted a large multiwavelength legacy effort exploiting its proximity to learn about starburst astrophysics. This Chandra legacy survey, approved for AO11, will provide an unprecendented vision of the anatomy of a massive, largely coherent mode of star formation, allowing detailed archeology of its history and content. 8000-10000 stars down to 0.5Modot and beyond will be detected, providing large samples to test theories of giant molecular cloud collapse, massive starformation and protoplanetary disk evolution.

Chandra Observation of Polaris: Census of Low-Mass Companions

Nancy Remage Evans, SAO
Scott Wolk (SAO), Edward Guinan (Villanova), Scott Engle (Villanova), Eric Schlegel (U. Texas, San Antonio), Brian Mason (USNO), Margarita Karovska (SAO), and Bradley Spitzbart (SAO)

We have observed Polaris = HD 8890 with Chandra ACIS-I for 10 ksec and found an X-ray source at the location of the Cepheid Polaris A = Aa + Ab with log LX = 28.89 ergs s-1 and a kT = 0.6 keV. Either the F7 Ib supergiant or the F6 V spectroscopic companion could produce a spectrum this soft. Polaris B is not an X-ray source, which is consistent with its early F spectral type. However, this shows that it does not itself have a lower mass companion. Two resolved low mass stars “C and D&8221; are not physical companions based on the lack of X-rays (indicating an older age than the Cepheid) and inconsistent motion. In addition, a possible more distant companion is identified, and also less plausible one. Thus, this observation provides a complete census of companions down to masses as small as an order of magnitude less than the Cepheid and nearly 15 mag fainter within the surrounding 0.1 pc.Funding for this work was provided by Chandra grant GO6-7011A Chandra X-ray Center NASA Contract NAS8-39073

High-energy processes in Young Stellar Objects - the radio-X-ray (dis)connection

Jan Forbrich, Harvard-Smithsonian Center for Astrophysics
Scott Wolk (CfA), Rachel Osten (STScI)

Low-mass young stellar objects show high levels of magnetic activity in a wide spectral range. Powerful flares have been observed from X-ray to radio wavelengths. It has been expected that radio and X-ray emission from YSOs are correlated if magnetic fields close to the star are responsible for both nonthermal radio emission (usually gyrosynchrotron radiation) and thermal hot-plasma X-ray emission (see Guedel & Benz 1994). These high-energy processes strongly influence the surroundings of the YSOs, including irradiation of their disks. A deeper understanding of these processes requires taking into account their manifestations in different spectral ranges. However, the strong variability of YSOs ideally necessitates simultaneous multi-wavelength observations or at least a large sample of sources. While a general correlation of radio and X-ray luminosities of phenomena ranging from solar flares to active stars has been found for more evolved stars, it remains unclear to what degree it applies to YSOs - particularly their earliest evolutionary stages. Drawing from the latest simultaneous X-ray and radio observations of star-forming regions as well as on archival data from the Chandra Orion Ultra-deep project, we present an update on the question of whether and how the radio and X-ray properties of YSOs are correlated and what this tells us about high-energy processes in YSOs compared to other classes of active stars. We mostly find a very limited relation between the X-ray and radio fluxes indicating a non-magnetic origin for some of the radio or X-ray emission.

Nothing to Hide - An X-ray Survey of Star Formation Activity in the Pipe Nebula

Jan Forbrich, Harvard-Smithsonian Center for Astrophysics
Bettina Posselt (CfA), Charles J. Lada (CfA), Kevin Covey (CfA/Cornell)

The Pipe Nebula, a large nearby molecular cloud, lacks obvious signposts of star formation in all but one of more than 130 dust extinction cores that have been identified within it. In a recent mid-infrared survey using Spitzer-MIPS to cover 13 square degrees, we have established that the star formation efficiency for the entire cloud is only ~0.06%. The mid-infrared data are most sensitive for the earliest evolutionary stages of Young Stellar Objects (YSOs), covering class I protostars and typical class II sources (classical T Tauri stars). X-ray observations allow us to extend our survey to constrain any population of classical and weak-line T Tauri stars. In a first step, we use the ROSAT All-Sky Survey to constrain any overall T Tauri star population of the Pipe Nebula. Due to the fact that the Pipe Nebula is at a distance of only 130 pc, the ROSAT survey is already quite sensitive. Assuming a typical level of extinction, the completeness for G- and K-type stars is estimated to be about 50%. Subsequently, we use XMM-Newton observations pointed at three high-extinction regions within the Pipe Nebula to analyze these areas at higher sensitivity. These three regions are Barnard 59, the only core with ongoing star formation, the “ring” (i.e., the highest extinction region in the “bowl” of the Pipe), and Barnard 68. We additionally analyze the YSOs of Barnard 59 in the radio continuum to constrain high-energy processes. Overall, our results corroborate our previous Spitzer result that the star formation efficiency of the Pipe Nebula is very low.

The Spitzer/Chandra View of the Triggered Star Formation in the CepB/OB3b Region

Konstantin Getman, Penn State University
Eric Feigelson (PSU), Kevin Luhman (PSU), Aurora Sicilia-Aguilar (Max-Planck-Institut fur Astronomie), Junfeng Wang (CfA), Gordon Garmire (PSU)

In this very active star forming complex which consists of the molecular bright-rimmed cloud CepB, the nearby OB association CepOB3b, and the HII interface S155, our Spitzer-Chandra study discovers a several hundred young stellar members. The analyses of the identified nearly disk-unbiased and mass complete samples of these young stars give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core toward the primary ionizing O star HD 217086. This strongly supports the radiation-driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of 1 km/s is very similar to theoretical models of RDI in shocked molecular clouds. The initial mass function (IMF) of the lightly obscured triggered population exhibits a standard Galactic field IMF shape. The unusually high apparent value of 70% star formation efficiency inferred from the ratio of star mass to current molecular gas mass indicates that most of the CepB molecular cloud has been already ablated or transformed to stars. Contrary to the current RDI simulations, our findings indicate that star formation triggering by HII region shocks is not restricted to a single episode but can continue for millions of years. Other results include: (1) agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; (2) confirmation of the youthfulness of the embedded Cep B cluster; (3) confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.

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X-ray Sources in the Rich Open Cluster NGC 6819

Natalie Gosnell, University of Wisconsin - Madison
P. Frinchaboy (Texas Christian University), A. Geller (UW - Madison), J. Kalirai (Space Telescope Science Institute), R. Mathieu (UW - Madison), D. Pooley (UW - Madison), E. Ramirez-Ruiz (UC - Santa Cruz)

We present the first study of the X-ray population of the intermediate-age rich open cluster NGC 6819, using the XMM-Newton Observatory. In the past decade, Chandra X-ray observations have shown a relationship between the X-ray population of globular clusters and their internal dynamics and encounter frequency. We will investigate whether a similar link exists in open clusters, utilizing X-ray and UV data from XMM-Newton along with the wealth of photometry and radial velocity data from the WIYN Open Cluster Study (WOCS). These preliminary results for the X-ray sources in NGC 6819 are the first in a group of 8 open clusters spanning a range of age and metallicities. We detect 76 X-ray sources in the field of NGC 6819 down to a luminosity of 1030 ergs/s for cluster members. Sources are classified by taking into account their broadband X-ray/UV/optical spectral properties, determining what types of X-ray sources have formed in NGC 6819 and ultimately whether or not they are primordial or linked to dynamics.

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A Smoking Gun in the Carina Nebula

Kenji Hamaguchi, NASA/GSFC & UMBC
M. F. Corcoran (NASA/GSFC & USRA), Y. Ezoe (Tokyo Metropolitan Univ.), L. Townsley (Pennsylvania State Univ), P. Broos (Pennsylvania State Univ), R. Gruendl (University of Illinois), K. Vaidya (University of Illinois), S. M. White (University of Maryland), T. Strohmayer (NASA/GSFC), R. Petre (NASA/GSFC), Y.-H. Chu (University of Illinois)

Massive stars are born from giant molecular clouds along with many lower mass stars, forming a stellar cluster or association. They dominate the pressure of the interstellar gas through their strong UV radiation, stellar winds and, ultimately, supernova explosions at the end of their life. These processes help the formation of the next generation of stars, but this trigger of star formation is not yet well understood. The Carina Nebula is one of the youngest, most active sites of massive star formation in our Galaxy. In this nebula, we have discovered a bright X-ray source that has persisted for ~30 years. The soft X-ray spectrum, consistent with a kT ~128 eV blackbody with mild extinction, and no counterpart in the optical and infrared wavelengths indicate that it is a 106 year-old neutron star. Current star formation theory does not allow the progenitor of the neutron star and the other massive stars in the Carina Nebula (in particular Eta Carinae) to be coeval. This result suggests that the Carina Nebula experienced at least two episodes of massive star formation. The neutron star may be responsible for part or all of the diffuse X-ray emission which permeates the Nebula.

Chandra Finds Buried Stellar Treasure in the Clouds of W40

Michael Kuhn, Penn State University
Konstantin Getman (PSU), Eric Feigelson (PSU), Bo Reipurth (IfA), Steven Rodney (IfA), Gordon Garmire (PSU)

The heavily obscured cluster illuminating the W40 HII region, one of the nearest massive star forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration this is a poorly studied stellar cluster with only a handful of bright stars identified in the optical band, including three OB stars as primary excitation sources. This neglected region deserves detailed investigation. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Our X-ray luminosity function (XLF) analysis confirms the 600 pc distance to W40. Under the assumption of a coeval population with a uniform obscuration the XLF analysis further gives a total stellar population of at least 400 stars down to 0.1Mo. However, we find spatial patches with different source obscurations and Ks-band-excess disk fractions which might suggest that the true underlying population is higher. The location of the strip with highest obscuration is unusual relative to the location of the central cluster and the cold molecular core. The disk fraction has a large variation across the field. Properties of the interesting sources, including diffuse X-ray emission, OB stars, and the young stellar object with a super-hot flare are also discussed.

The population of young stars in Orion A: X-rays and IR properties.

Ignazio Pillitteri, SAO - Harvard Center for Astrophysics
S. Wolk (SAO - Harvard Center for Astrophysics), L. Allen (National Optical Astronomy Observatory),T. Megeath (Department of Physics & Astronomy, University of Toledo), and SOXS collaboration

Stars in the very early stages of their formation are characterized by strong IR excess and X-rays emission. We present the results of the survey of Orion A in both IR and X-rays obtained with Spitzer and XMM-Newton observatories. We study the SED Class of the Young Stellar Objects (YSOs) population using IR colors from 2MASS + Spitzer (IRAC & MIPS) and by means of X-rays luminosities and plasma temperatures. We discuss clustering properties and spatial segregation among different IR class YSOs in order to track the formation history.

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Modeling accretion shocks on CTTSs and their X-ray emission

Giuseppe Sacco, Rochester Institute of Technology
Salvatore Orlando (INAF/OAPA), Costanza Argiroffi (UNIPA/DSFA), Antonio Maggio (INAF/OAPA), Giovanni Peres (UNIPA/DSFA), Fabio Reale (UNIPA/DSFA)

Recent high spectral resolution X-ray observations of some CTTSs show the presence of high density plasma (ne=1011-1013 cm-3) at temperature T=2-3 MK. This plasma is likely heated up by an accretion shock on the star surface. We investigate this issue by an accurate modelling of the impact of an accretion stream onto the stellar chromosphere. Specifically, we present a large set of 1D hydrodynamical simulations aimed at investigating the physical properties of the system as a function of the density, and the velocity of the accretion stream and of the abundances of the heavy elements. We also synthesize the plasma X-ray emission from the simulations results, in order to link the observed properties of the CTTSs with the accretion shock physics. Furthermore, we present results of 2D magneto-hydrodynamical simulations aimed at investigating the role of the magnetic field in confining the shocked plasma at the base of the accretion column for different magnetic field strength.

Chandra observations of the massive star forming region, Cygnus OB2

Nicholas Wright, Harvard-Smithsonian Center for Astrophysics
Jeremy Drake (Harvard-Smithsonian Center for Astrophysics)

We present results from a Chandra study of the massive star forming region, Cygnus OB2. At a distance of 1.5kpc, Cygnus OB2 is the nearest massive star forming region, hosting thousands of OB stars and millions of low-mass stars. It is therefore an ideal location to study how the star formation process varies in such extreme environments. We combine Chandra X-ray observations with photometry from recent deep Galactic Plane surveys to study the stellar population down to sub-solar masses. We detect ~1700 X-ray sources, the majority of which have optical or near-IR associations. Near-IR photometry is used to study the age and structure of the star forming region. Evidence for an older generation of star formation is presented, and we discuss this in terms of the history of the region. The fraction of sources with inner circumstellar disks is found to be small at only 5-10%, which is interpreted as due to both the influence of the large number of OB stars in the region and the presence of slightly older stars in the region. The initial mass function shows a Salpeter slope down to our completeness limit and provides no evidence for a flatter slope at high masses as has been suggested for massive star forming regions. The spatial distribution of sources shows a small amount of structure, suggesting the cluster is dynamically evolved and a two-point correlation function is used to estimate the age and velocity dispersion in the region. A fractured minimum spanning tree is used to provide evidence for mass segregation, which is considered in terms of the dynamical evolution of the region. Finally these results are discussed in terms of the age and star formation history of Cygnus OB2 and the implications for the wider evolution of the Cygnus X region.