arXiv subjects

Eric D. Feigelson

Publications and source records attributed to Eric D. Feigelson.

At least 109 records · Page 6Linked to original sources

arXiv2006

A Re-appraisal of the Habitability of Planets Around M Dwarf Stars

Stable, hydrogen-burning, M dwarf stars comprise about 75% of all stars in the Galaxy. They are extremely long-lived and because they are much smaller in mass than the Sun (between 0.5 and 0.08 MSun), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially-habitable, wet planets residing within their habitable zones, which are only ~ 1/5 to 1/50 of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 MEarth orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone doesn't necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity, or thermal and non-thermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life.

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arXiv2006

X-ray Study of Triggered Star Formation and Protostars in IC 1396N

The IC 1396N cometary globule within the large nearby HII region IC 1396 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler~37 dispersed throughout the HII region, and 25 are associated with young stars formed within the globule. Spitzer/2MASS photometry shows the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the undetected population, with a star formation efficiency of 1-4%. An elongated source spatial distribution with an age gradient oriented towards the exciting star is discovered in the X-ray population of IC 1396N, supporting similar findings in other cometary globules. The geometric and age distribution is consistent with the RDI model for triggered star formation in CGs by HII region shocks. The inferred velocity of the shock front propagating into the globule is ~0.6km/s. The large number of X-ray-luminous protostars in the globule suggests either an unusually high ratio of Class I/0 vs. Class II/III stars, or a non-standard IMF favoring higher mass stars by the triggering process. The Chandra source associated with the luminous Class 0/I protostar IRAS 21391+5802 is one of the youngest stars ever detected in the X-ray band. We also establish for the first time that the X-ray absorption in protostars arises from the local infalling envelopes rather than ambient molecular cloud material.

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arXiv2006

An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357

We present the first high spatial resolution X-ray study of the massive star forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large HII region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and Initial Mass Function and detect ~800 X-ray sources with a limiting sensitivity of 10^{30} ergs s^{-1}; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high luminosity end (log L_h[2-8 keV]\ge 30.3 ergs s^{-1}) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing Lx - 10^{-7}L_{bol} correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of 1 Myr), but only a few exhibit K-band excess. We report the first detection of X-ray emission from an Evaporating Gaseous Globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

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arXiv2006

X-rays from HH210 in the Orion nebula

We report the detection during the Chandra Orion Ultradeep Project (COUP) of two soft, constant, and faint X-ray sources associated with the Herbig-Haro object HH210. HH210 is located at the tip of the NNE finger of the emission line system bursting out of the BN-KL complex, northwest of the Trapezium cluster in the OMC-1 molecular cloud. Using a recent Halpha image obtained with the ACS imager on board HST, and taking into account the known proper motions of HH210 emission knots, we show that the position of the brightest X-ray source, COUP703, coincides with the emission knot 154-040a of HH210, which is the emission knot of HH210 having the highest tangential velocity (425 km/s). The second X-ray source, COUP704, is located on the complicated emission tail of HH210 close to an emission line filament and has no obvious optical/infrared counterpart. Spectral fitting indicates for both sources a plasma temperature of ~0.8 MK and absorption-corrected X-ray luminosities of about 1E30 erg/s (0.5-2.0 keV). These X-ray sources are well explained by a model invoking a fast-moving, radiative bow shock in a neutral medium with a density of ~12000 cm^{-3}. The X-ray detection of COUP704 therefore reveals, in the complicated HH210 region, an energetic shock not yet identified at other wavelengths.

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arXiv2006

Chandra Study of the Cepheus B Star Forming Region: Stellar Populations and the Initial Mass Function

Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB association, one of the most active regions of star formation within 1 kpc, has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 431 X-ray sources, of which 89% are confidently identified as clustered pre-main sequence stars. Two main results are obtained. First, we provide the best census to date for the stellar population of the region. We identify many members of two rich stellar clusters: the lightly obscured Cep OB3b association, and the deeply embedded cluster in Cep B whose existence was previously traced only by a handful of radio sources and T Tauri stars. Second, we find a discrepancy between the X-ray Luminosity Functions of the Cep OB3b and the Orion Nebula Cluster. This may be due to different Initial Mass Functions of two regions (excess of ~0.3 solar mass stars), or different age distributions. Several other results are obtained. A diffuse X-ray component seen in the field is attributed to the integrated emission of unresolved low mass PMS stars. The X-ray emission from HD 217086 (O7n), the principle ionizing source of the region, follows the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. The X-ray source #294 joins a number of similar superflare PMS stars where long magnetic structures may connect the protoplanetary disk to the stellar surface.

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arXiv2005

X-ray Emission from Orion Nebula Cluster Stars with Circumstellar Disks and Jets

We investigate the X-ray and near-infrared emission properties of a sample of pre-main sequence (PMS) stellar systems in the Orion Nebula Cluster (ONC) that display evidence for circumstellar disks (``proplyds'') and optical jets in Hubble Space Telescope (HST) imaging. Approximately 70% of ~140 proplyds were detected as X-ray sources in the Chandra Orion Ultradeep Program observation of the ONC, including ~25% of proplyds that do not display central stars in HST imaging. Many proplyds display near-infrared excesses, suggesting disk accretion is ongoing onto the central, PMS stars. Among circumstellar disks that are detected in absorption in HST imaging, we find that X-ray absorbing column and apparent disk inclination are well correlated, providing insight into the disk scale heights and metal abundances of UV- and X-ray-irradiated protoplanetary disks. Jet sources display some of the largest near-infrared excesses among the X-ray-detected proplyds, suggesting that the origin of the jets is closely related to ongoing, PMS stellar accretion. Some jet sources display double-peaked X-ray spectral energy distributions, with soft components indicative of strong shocks in the jet collimation region. These results support models in which circumstellar disks collimate and/or launch jets from young stellar objects and, furthermore, demonstrate that star-disk-jet interactions may contribute to PMS X-ray emission.

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arXiv2005

The Origin of T Tauri X-ray Emission: New Insights from the Chandra Orion Ultradeep Project

We use the data of the Chandra Orion Ultradeep Project (COUP) to study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. We detect X-ray emission from more than 97% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no ``X-ray quiet'' population of late-type stars with suppressed magnetic activity. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity rises slowly with mass over the 0.1 - 2 M_sun range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The large scatter about the relations between X-ray activity and stellar parameters seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of ~2-3 on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars.

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arXiv2005

Global X-ray properties of the Orion Nebula region

Based on the Chandra Orion Ultradeep Project (COUP) observation, we establish the global X-ray properties of the stellar population associated with the Orion Nebula. Three components contribute roughly equally to the integrated COUP luminosity in the hard (2-8 keV) X-ray band: several OB stars, 822 lightly obscured cool stars in the Orion Nebula Cluster (ONC), and 559 heavily obscured stars. ONC stars 0.5-2 pc from the center show a spatial asymmetry consistent with violent relaxation in the stellar dynamics. The obscured COUP sources concentrate around both OMC-1 molecular cores; these small-scale structures indicate ages t < 0.1 Myr. The X-ray luminosity function (XLF) of the lightly obscured sample is roughly lognormal in shape. The obscured population is deficient in lower-luminosity stars, perhaps due to localized circumstellar material. Mass-stratified XLFs show that one-third of the Orion Nebula region hard-band emission is produced by the bright O6 star theta-1 Ori C and half is produced by lower mass pre-main sequence stars with masses 0.3 < M < 3 Mo. Very low mass stars contribute little to the cluster X-ray emission. Using the hard band emission, we show that young stellar clusters like the ONC can be readily detected and resolved with Chandra across the Galactic disk, even in the presence of heavy obscuration. The Orion Nebula sample is a valuable template for studies of distant clusters. For example, the peak of the XLF shape can serve as a standard candle for a new distance measure to distant young stellar clusters, and the presence of a neon emission line complex around 1 keV can serve as a diagnostic for young stars.

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arXiv2005

The evolution of X-ray emission in young stars

We study the relation between age and magnetic activity in late-type pre-main sequence (PMS) stars, for the first time using mass-stratified subsamples. The effort is based on the Chandra Orion Ultradeep Project (COUP) which provides very sensitive and homogenous X-ray data on a uniquely large sample of 481 optically well-characterized low-extinction low-mass members of the Orion Nebula Cluster, for which individual stellar masses and ages could be determined. More than 98 percent of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range $\sim 0.1-10$ Myr, we establish a mild decay in activity with stellar age $τ$ roughly as $L_{\rm X} \propto τ^{-1/3}$. On longer timescales, when the Orion stars are compared to main sequence stars, the X-ray luminosity decay law for stars in the $0.5 < M < 1.2$ M$_\odot$ mass range is more rapid with $L_{\rm X} \propto τ^{-0.75}$ over the wide range of ages $5 < \log τ< 9.5$ yr. The magnetic activity history for M stars with masses $0.1 < M < 0.4 M_\odot$ is distinctly different. Only a mild decrease in X-ray luminosity, and even a mild increase in $L_{\rm X}/L_{\rm bol}$ and $F_{\rm X}$, is seen over the 1-100 Myr range, though the X-ray emission does decay over long timescales on the main sequence. Together with COUP results on the absence of a rotation-activity relation in Orion stars, we find that the activity-age decay is strong across the entire history of solar-type stars but is not attributable to rotational deceleration during the early epochs. A combination of tachocline and distributed convective dynamos may be operative in young solar-type stars. The results for the lowest mass stars are most easily understood by the dominance of convective dynamos during both the PMS and main sequence phases.

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arXiv2005

X-ray emission from young brown dwarfs in the Orion Nebula Cluster

We use the sensitive X-ray data from the Chandra Orion Ultradeep Project (COUP) to study the X-ray properties of 34 spectroscopically-identified brown dwarfs with near-infrared spectral types between M6 and M9 in the core of the Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray sources. The apparently low detection rate is in many cases related to the substantial extinction of these brown dwarfs; considering only the BDs with $A_V \leq 5$ mag, nearly half of the objects (7 out of 16) are detected in X-rays. Our 10-day long X-ray lightcurves of these objects exhibit strong variability, including numerous flares. While one of the objects was only detected during a short flare, a statistical analysis of the lightcurves provides evidence for continuous (`quiescent') emission in addition to flares for all other objects. Of these, the $\sim$ M9 brown dwarf COUP 1255 = HC 212 is one of the coolest known objects with a clear detection of quiescent X-ray emission. The X-ray properties (spectra, fractional X-ray luminosities, flare rates) of these young brown dwarfs are similar to those of the low-mass stars in the ONC, and thus there is no evidence for changes in the magnetic activity around the stellar/substellar boundary, which lies at $\sim$ M6 for ONC sources. Since the X-ray properties of the young brown dwarfs are also similar to those of M6--M9 field stars, the key to the magnetic activity in very cool objects seems to be the effective temperature, which determines the degree of ionization in the atmosphere.

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arXiv2005

X-rays and young clusters: Membership, IMFs and distances

Sensitive imaging X-ray observations of young stellar clusters (YSCs, ages less than about 10 Myr) are valuable tools for the acquisition of an unbiased census of cluster members needed for Initial Mass Function (IMF) studies. Several dozen YSCs, both nearby and across the Galactic disk, have been observed with the Chandra and XMM-Newton satellites, detecting >10,000 low-mass cluster members. Many of these samples should be nearly complete down to about 1 Mo. An important additional benefit is that the YSC X-ray luminosity function appears to be universal with a lognormal shape, providing a new standard candle for measurement of YSC distances.

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arXiv2004

An X-ray census of young stars in the Chamaeleon I North cloud

Sensitive X-ray imaging surveys provide a new and effective tool to establish the census of pre-main sequence (PMS) stars in nearby young stellar clusters. We report here a deep Chandra X-ray Observatory observation of PMS stars in the Chamaeleon I North cloud, achieving a limiting luminosity of log L_t ~ 27 erg/s (0.5-8 keV band) in a 0.8x0.8 pc region. Of the 107 X-ray sources, 37 are associated with Galactic stars of which 27 are previously recognized cloud members. These include three PMS brown dwarfs; the protostellar brown dwarf ISO 192 has a particularly high level of magnetic activity. Followup optical photometry and spectroscopy establishes that 9-10 of the Chandra sources are probably magnetically active background stars. Several previously proposed cloud members are also inferred to be interlopers due to the absence of X-ray emission at the level expected from the L_t - K correlation. No new X-ray discovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in the Chandra field is uncontaminated and complete down to K = 12 or M ~ 0.1 Mo. The initial mass function (IMF) derived from our sample is deficient in 0.1-0.3 Mo stars compared to the IMF of the rich Orion Nebula Cluster and other Galactic populations. We can not discriminate whether this is due to different star formation processes, mass segregation, or dynamical ejection of lower mass stars.

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arXiv2004

Chandra/ACIS Subpixel Event Repositioning. II. Further Refinements and Comparison between Backside and Front-side Illuminated X-ray CCDs

We further investigate subpixel event repositioning (SER) algorithms in application to Chandra X-ray Observatory (CXO) CCD imaging. SER algorithms have been applied to backside illuminated (BI) Advanced CCD Imaging Spectrometer (ACIS) devices, and demonstrate spatial resolution improvements in Chandra/ACIS observations. Here a new SER algorithm that is charge split dependent is added to the SER family. We describe the application of SER algorithms to frontside illuminated (FI) ACIS devices. The results of SER for FI CCDs are compared with those obtained from SER techniques applied to BI CCD event data. Both simulated data and Chandra/ACIS observations of the Orion Nebular Cluster were used to test and evaluate the achievement of the various SER techniques.

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arXiv2003

Population and dynamical state of the eta Chamaeleontis sparse young open cluster

We report new results in our continuing study of the unique compact (1 pc extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the B9 star eta Chamaeleontis. An optical photometric survey spanning 1.3 x 1.3 pc adds two M5-M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses > 0.15 M_sun. The cluster now consists of 17 primaries and approximately 9 secondaries lying within 100 AU of their primaries. The apparent distribution of 9:7:1 single:binary:triple systems shows 2-4 x higher multiplicity than in the field main sequence stars, and is comparable to that seen in other pre-main sequence (PMS) populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10-14 additional low mass stars with 0.08 < M < 0.15 M-sun and 10-15 brown dwarfs with 0.025 < M < 0.08 M_sun remain to be discovered. The eta Cha cluster extends the established stellar density and richness relationship for young open clusters. The radial distribution of stars is consistent with an isothermal sphere, but mass segregation is present with > 50 percent of the stellar mass residing in the inner 6 arcmin (0.17 pc). Considering that the eta Cha cluster is sparse, diffuse and young, the cluster may be an ideal laboratory for distinguishing between mass segregation that is primordial in nature, or arising from dynamical interaction processes.

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arXiv2003

The epsilon Chamaeleontis young stellar group and the characterization of sparse stellar clusters

We present the outcomes of a Chandra X-ray Observatory snapshot study of five nearby Herbig Ae/Be (HAeBe) stars which are kinematically linked with the Oph-Sco-Cen Association (OSCA). Optical photometric and spectroscopic followup was conducted for the HD 104237 field. The principal result is the discovery of a compact group of pre-main sequence (PMS) stars associated with HD 104237 and its codistant, comoving B9 neighbor epsilon Chamaeleontis AB. We name the group after the most massive member. The group has five confirmed stellar systems ranging from spectral type B9-M5, including a remarkably high degree of multiplicity for HD 104237 itself. The HD 104237 system is at least a quintet with four low mass PMS companions in nonhierarchical orbits within a projected separation of 1500 AU of the HAeBe primary. Two of the low-mass members of the group are actively accreting classical T Tauri stars. The Chandra observations also increase the census of companions for two of the other four HAeBe stars, HD 141569 and HD 150193, and identify several additional new members of the OSCA. We discuss this work in light of several theoretical issues: the origin of X-rays from HAeBe stars; the uneventful dynamical history of the high-multiplicity HD 104237 system; and the origin of the epsilon Cha group and other OSCA outlying groups in the context of turbulent giant molecular clouds. Together with the similar eta Cha cluster, we paint a portrait of sparse stellar clusters dominated by intermediate-mass stars 5-10 Myr after their formation.

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arXiv2003

Coronal X-ray emission from an intermediate-age brown dwarf

We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in a $\simeq 12$ Myr old pre-main sequence binary system. We clearly resolve the faint companion (35 photons) separated from the X-ray luminous primary by 2 arcsec in a {\it Chandra} ACIS image. TWA 5B shows a soft X-ray spectrum with a low plasma temperature of only 0.3 keV and a constant flux during the 3 hour observation, of which the characteristics are commonly seen in the solar corona. The X-ray luminosity is 4$\times10^{27}$ erg s$^{-1}$ (0.1--10 keV band) or $\log L_X/L_{bol} = -3.4$. Comparing these properties to both younger and older BDs, we discuss the evolution of the X-ray emission in BDs. During their first few Myr, they exhibit high levels of X-ray activity as seen in higher mass pre-main sequence stars. The level in TWA 5B is still high at $t \simeq 12$ Myr in $\log L_X/L_{bol}$ while $kT$ has already substantially cooled.

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arXiv2002

X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars

A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075 sources (Feigelson et al. 2002), providing a uniquely large and well-defined sample to study the dependence of magnetic activity on bulk properties for stars descending the Hayashi tracks. The following results are obtained: (1) X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with bolometric luminosity with = -3.8 for stars with masses 0.7<M<2 Mo, an order of magnitude below the main sequence saturation level; (2) the X-ray emission drops rapidly below this level in some or all stars with 2<M<3 Mo; (3) the presence or absence of infrared circumstellar disks has no apparent relation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise as rotational periods increase from 0.4 to 20 days. This last finding stands in dramatic contrast to the strong anticorrelation between X-rays and period seen in main sequence stars. The absence of a strong X-ray/rotation relationship in PMS stars, and particularly the high X-ray values seen in some very slowly rotating stars, is a clear indication that the mechanisms of magnetic field generation differ from those operating in main sequence stars. The most promising possibility is a turbulent dynamo distributed throughout the deep convection zone, but other models such as alpha-Omega dynamo with `supersaturation' or relic core fields are not immediately excluded. The drop in magnetic activity in intermediate-mass stars may reflect the presence of a significant radiative core. The evidence does not support X-ray production in large-scale star-disk magnetic fields.

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arXiv2002

Infrared study of the eta Chamaeleontis cluster and the longevity of circumstellar discs

We have analyzed JHKL observations of the stellar population of the ~9 Myr-old eta Chamaeleontis cluster. Using infrared (IR) colour-colour and colour-excess diagrams, we find the fraction of stellar systems with near-IR excess emission is 0.60 pm 0.13 (2_sigma). This results implies considerably longer disc lifetimes than found in some recent studies of other young stellar clusters. For the classical T Tauri (CTT) and weak-lined T Tauri (WTT) star population, we also find a strong correlation between the IR excess and H_alpha emission. The IR excesses of these stars indicate a wide range of star-disc activity; from a CTT star showing high levels of accretion, to CTT - WTT transition objects with evidence for some on-going accretion, and WTT stars with weak or absent IR excesses. Of the 15 known cluster members, 4 stars with IR excesses delta(K-L) > 0.4 mag are likely experiencing on-going accretion owing to strong or variable optical emission. The resulting accretion fraction (0.27 pm 0.13; 2_sigma) shows that the accretion phase, in addition to the discs themselves, can endure for at least ~10 Myr.

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