Long-lived Disks Research Articles

Eccentric signatures of stellar-mass binary black holes with circumbinary disks in LISA

Abstract Stellar-mass binary black holes may have circumbinary disks if formed through common-envelope evolution or within gaseous environments. Disks can drive binaries into wider and more eccentric orbits, while gravitational waves harden and circularise them. We combine cutting-edge evolution prescriptions for disk-driven binaries with well-known equations for gravitational-wave-driven evolution, and study the evolution of stellar-mass binary black holes. We find that binaries are driven by their disk to an equilibrium eccentricity, 0.2 ≲ eeq ≲ 0.5, that dominates their evolution. Once they transition to the GW-dominated regime their eccentricity decreases rapidly; we find that stellar-mass binary black holes with long-lived disks will likely be observed in LISA with detectable eccentricities ∼10−2 at 0.01 Hz, with the precise value closely correlating with the binary’s initial mass ratio. This may lead stellar-mass binary black holes with CBDs observed in LISA to be confused with dynamically-formed binary black holes.

How accretion of planet-forming disks influences stellar abundances

Millimeter-sized dust grains experience radial velocities exceeding the gas velocities by orders of magnitude. The viscous evolution of the accretion disk adds disk material onto the central star’s convective envelope, influencing its elemental abundances, [X/H]. At the same time, the envelope mass shrinks as the stellar age increases, amplifying the rate of abundance change. Therefore, the elemental abundances of the star are sensitive to disk processes that alter the composition and timing of disk accretion. We performed numerical 1D log-radial simulations integrating the disk advection-diffusion equation, while accounting for evaporation and condensation of chemical species at the evaporation fronts. They reveal a peak of refractory abundance within the first 2 Myr of Δ[X/H] ~ 5 × 10−2 if grain growth is significant, but subsequent accretion diminishes previous refractory abundance increases for long-lived disks. Planet formation can reduce the abundance of dust species whose evaporation fronts lie within the planet’s orbit by preventing solids from reaching the inner edge once the planet starts opening a gap exerting a pressure bump exterior to its orbit and consequently blocking inward drifting pebbles. We expect the accretion of the solar protoplanetary disk with Jupiter present to have changed the Sun’s elemental abundances by ~1 × 10−2 throughout its lifetime. These considerations were also applied to the HD106515 wide binary system. We find that measurements of Δ[X/H] are in reasonable agreement with results from simulations where the observed giant planet around HD106515 A is included and if HD106515B’s disk formed planetesimals more efficiently. Simulations where the planet formed inside the water ice line are more favorable to agree with observations. Even though the general changes in the stellar abundances due to disk accretion are small, they are detectable at current sensitivities, indicating that the methods presented here can be used to constrain the planet formation pathway.

Late-time accretion in neutron star mergers: Implications for short gamma-ray bursts and kilonovae

ABSTRACT We study the long-term ($t\gg 10\rm \, s\!$ ) evolution of the accretion disc after a neutron star(NS)–NS or NS–black hole merger, taking into account the radioactive heating by r-process nuclei formed in the first few seconds. We find that the cumulative heating eventually exceeds the disc’s binding energy at $t\sim 10^2\mathrm{\, s}\, (\alpha /0.1)^{-1.8}(M/2.6\, \mathrm{M}_{\odot })^{1.8}$ after the merger, where α is the viscosity parameter and M is the mass of the remnant object. This causes the disc to evaporate rapidly and the jet power to shut off. We propose that this corresponds to the steep flux decline at the end of the extended emission (EE) or X-ray plateau seen in many short gamma-ray bursts (GRBs). The shallow flux evolution before the steep decline is consistent with a plausible scenario where the jet power scales linearly with the disc mass. We suggest that the jets from NS mergers have two components – a short-duration narrow one producing the prompt gamma-rays and a long-lasting wide component producing the EE. This leads to a prediction that ‘orphan EE’ (without short GRB) may be a promising electromagnetic counterpart for NS mergers observable by future wide-field X-ray surveys. The long-lived disc produces a slow ejecta component that can efficiently thermalize the β-electrons’ energy up to $t\sim 100\rm \, d$ and contributes $\sim \!10~{{\ \rm per\ cent}}$ of the kilonova’s bolometric luminosity at these late epochs. We predict that future ground-based and JWST near-IR spectroscopy of nearby ($\lesssim 100\rm \, Mpc\!$ ) NS mergers will detect narrow (Δv ∼ 0.01c) line features a few weeks after the merger.

On the Unusual Variability of 2MASS J06195260–2903592: A Long-lived Disk around a Young Ultracool Dwarf

We present the characterization of the low-gravity M6 dwarf 2MASS J06195260-2903592, previously identified as an unusual field object based on its strong IR excess and variable near-IR spectrum. Multiple epochs of low-resolution (R ≈ 150) near-IR spectra show large-amplitude (≈0.1–0.5 mag) continuum variations on timescales of days to 12 yr, unlike the small-amplitude variability typical for field ultracool dwarfs. The variations between epochs are well-modeled as changes in the relative extinction (ΔA V ≈ 2 mag). Similarly, Panoramic Survey Telescope and Rapid Response System 1 optical photometry varies on timescales as long as 11 yr (and possibly as short as an hour) and implies comparable A V changes. Near Earth Object Wide-field Infrared Survey Explorer mid-IR light curves also suggest changes on 6 month timescales, with amplitudes consistent with the optical/near-IR extinction variations. However, near-IR spectra, near-IR photometry, and optical photometry obtained in the past year indicate that the source can also be stable on hourly and monthly timescales. From comparison to objects of similar spectral type, the total extinction of 2MASS J0619-2903 seems to be A V ≈ 4–6 mag, with perhaps epochs of lower extinction. Gaia Early Data Release 3 (EDR3) finds that 2MASS J0619-2903 has a wide-separation (1.′2 = 10,450 au) stellar companion, with an isochronal age of Myr and a mass of M ☉. Adopting this companion’s age and EDR3 distance (145.2 ± 0.6 pc), we estimate a mass of 0.11–0.17 M ☉ for 2MASS J0619-2903. Altogether, 2MASS J0619-2903 appears to possess an unusually long-lived primordial circumstellar disk, perhaps making it a more obscured analog to the “Peter Pan” disks found around a few M dwarfs in nearby young moving groups.

Dispersal of protoplanetary discs: how stellar properties and the local environment determine the pathway of evolution

ABSTRACT We study the evolution and final dispersal of protoplanetary discs that evolve under the action of internal and external photoevaporation, and different degrees of viscous transport. We identify five distinct dispersal pathways, which are (i) very long-lived discs ($\gt 20\,$ Myr), (ii) inside-out dispersal where internal photoevaporation dominates and opens inner holes, (iii) outside-in dispersal where external photoevaporation dominates through disc truncation, and two intermediate regimes characterized by lingering material in the inner disc with the outer disc dispersed predominantly by either internal or external photoevaporation. We determine how the lifetime, relative impact of internal and external winds, and clearing pathway vary over a wide, plausible, parameter space of stellar/disc/radiation properties. There are a number of implications, for example, in high UV environments because the outer disc lifetime is shorter than the time-scale for clearing the inner disc, we do not expect transition discs to be common, which appears to be reflected in the location of transition disc populations towards the Orion Nebular Cluster. Irrespective of environment, we find that ongoing star formation is required to reproduce observed disc fractions as a function of stellar cluster age. This work demonstrates the importance of including both internal and external winds for understanding protoplanetary disc evolution.

Planetesimals around stars with TESS (PAST) – II. An M dwarf ‘dipper’ star with a long-lived disc in the TESS continuous viewing zone

ABSTRACT Studies of T Tauri discs inform planet formation theory; observations of variability due to occultation by circumstellar dust are a useful probe of unresolved, planet-forming inner discs, especially around faint M dwarf stars. We report observations of 2M0632, an M dwarf member of the Carina young moving group that was observed by Transiting Exoplanet Survey Satellite over two 1-yr intervals. The combined light curve contains >300 dimming events, each lasting a few hours, and as deep as 40 per cent (0.55 magnitudes). These stochastic events are correlated with a distinct, stable 1.86-d periodic signal that could be stellar rotation. Concurrent ground-based, multiband photometry show reddening consistent with interstellar medium-like dust. The star’s excess emission in the infrared and emission lines in optical and infrared spectra reveal a T Tauri-like accretion disc around the star. We confirm membership of 2M0632 in the Carina group by a Bayesian analysis of its Galactic space motion and position. We combine stellar evolution models with Gaia photometry and constraints on Teff, luminosity, and the absence of detectable lithium in the photosphere to constrain the age of the group and 2M0632 to 40–60 Myr, consistent with earlier estimates. 2M0632 joins a handful of long-lived discs which challenge the canon that disc lifetimes are ≲10 Myr. All known examples surround M dwarfs, suggesting that lower X-ray/ultraviolet irradiation and slower photoevaporation by these stars can dramatically affect disc evolution. The multiplanet systems spawned by long-lived discs probably experienced significant orbital damping and migration into close-in, resonant orbits, and perhaps represented by the TRAPPIST-1 system.

Peter Pan discs: finding Neverland’s parameters

ABSTRACT Peter Pan discs are a recently discovered class of long-lived discs around low-mass stars that survive for an order of magnitude longer than typical discs. In this paper, we use disc evolutionary models to determine the required balance between initial conditions and the magnitude of dispersal processes for Peter Pan discs to be primordial. We find that we require low transport (α ∼ 10−4), extremely low external photoevaporation (${\le}10^{-9}\, {\rm M}_{\odot }\, {\rm yr^{-1}}$), and relatively high disc masses (>0.25M*) to produce discs with ages and accretion rates consistent with Peter Pan discs. Higher transport (α = 10−3) results in disc lifetimes that are too short and even lower transport (α = 10−5) leads to accretion rates smaller than those observed. The required external photoevaporation rates are so low that primordial Peter Pan discs will have formed in rare environments on the periphery of low-mass star-forming regions, or deeply embedded, and as such have never subsequently been exposed to higher amounts of UV radiation. Given that such an external photoevaporation scenario is rare, the required disc parameters and accretion properties may reflect the initial conditions and accretion rates of a much larger fraction of the discs around low-mass stars.

Candidate Wide-separation Companions to Nearby, Dusty Young Stars: Gaia Weighs In

We exploit the precise parallaxes and proper motions contained in Gaia Data Release 2 (DR2) to establish whether the proposed wide-separation companions to five nearby, young stars are in fact equidistant and comoving with these stars. For three of the proposed wide pairs --- TW Hya + 2M1102-34; HR 4796A + 2M1235-39; and V4046 Sgr AB + GSC 07396 --- the Gaia DR2 data confirm that the two stars lie at the same distance and are comoving (or nearly so), to within the measurement errors. We conclude that these three pairs indeed constitute wide-separation binaries with projected separations of 44.3 kau, 13.3 kau, and 12.3 kau, respectively. In contrast, the DR2 data disprove the hypothesis that T Cha + 2M1155-79 and HD 113766AB + TYC 8246-2900-1 constitute wide binaries. Future investigations of the three wide young-star pairs confirmed in this work should be aimed at establishing whether they remain bound at the present epoch and whether the presence of long-lived disks orbiting the primaries is linked to the orbital or dissolution timescales of these wide binaries.

ZOMG – II. Does the halo assembly history influence central galaxies and gas accretion?

The growth-rate and the internal dynamics of galaxy-sized dark-matter haloes depend on their location within the cosmic web. Haloes that sit at the nodes grow in mass till the present time and are dominated by radial orbits. Conversely, haloes embedded in prominent filaments do not change much in size and are dominated by tangential orbits. Using zoom hydrodynamical simulations including star formation and feedback, we study how gas accretes onto these different classes of objects that, for simplicity, we dub 'accreting' and 'stalled' haloes. We find that all haloes get a fresh supply of newly accreted gas in their inner regions, although this slowly decreases with time, in particular for the stalled haloes. The inflow of new gas is always higher than (but comparable with) that of recycled material. Overall, the cold-gas fraction increases (decreases) with time for the accreting (stalled) haloes. In all cases, a stellar disc and a bulge form at the centre of the simulated haloes. The total stellar mass is in excellent agreement with expectations based on the abundance-matching technique. Many properties of the central galaxies do not seem to correlate with the large-scale environment in which the haloes reside. However, there are two notable exceptions that characterise stalled haloes with respect to their accreting counterparts: i) the galaxy disc contains much older stellar populations; ii) its vertical scale-height is larger by a factor of two or more. This thickening is likely due to the heating of the long-lived discs by mergers and close flybys.

EVIDENCE FOR CLUSTER TO CLUSTER VARIATIONS IN LOW-MASS STELLAR ROTATIONAL EVOLUTION

ABSTRACT The concordance model for angular momentum evolution postulates that star-forming regions and clusters are an evolutionary sequence that can be modeled with assumptions about protostar–disk coupling, angular momentum loss from magnetized winds that saturates in a mass-dependent fashion at high rotation rates, and core-envelope decoupling for solar analogs. We test this approach by combining established data with the large h Per data set from the MONITOR project and new low-mass Pleiades data. We confirm prior results that young low-mass stars can be used to test star–disk coupling and angular momentum loss independent of the treatment of internal angular momentum transport. For slow rotators, we confirm the need for star–disk interactions to evolve the ONC to older systems, using h Per (age 13 Myr) as our natural post-disk case. There is no evidence for extremely long-lived disks as an alternative to core-envelope decoupling. However, our wind models cannot evolve rapid rotators from h Per to older systems consistently, and we find that this result is robust with respect to the choice of angular momentum loss prescription. We outline two possible solutions: either there is cosmic variance in the distribution of stellar rotation rates in different clusters or there are substantially enhanced torques in low-mass rapid rotators. We favor the former explanation and discuss observational tests that could be used to distinguish them. If the distribution of initial conditions depends on environment, models that test parameters by assuming a universal underlying distribution of initial conditions will need to be re-evaluated.

Herschel-PACS observations of discs in theηChamaeleontis association

Context. Protoplanetary discs are the birthplace of planets. Studying protoplanetary discs is the key to constraining theories of planet formation. By observing dust and gas in associations at different ages we can study the evolution of these discs, their clearing timescales, and their physical and geometrical properties.The stellar association η Cha is peculiar; some members still retain detectable amounts of gas in their discs at the late age of ~7 Myr, making it one of the most interesting young stellar associations in the solar neighbourhood. Aims: We characterise the properties of dust and gas in protoplanetary and transitional discs in the η Cha young cluster, with special emphasis on explaining the peculiarities that lead to the observed high disc detection fraction and prominent IR excesses at an age of ~7 Myr. Methods: We observed 17 members of the η Cha association with Herschel-PACS in photometric mode and line spectroscopic mode. A subset of members were also observed in range spectroscopic mode. The observations trace [OI] and H2O emissions at 63.18 and 63.32 μm, respectively, as well as CO, OH, CH+, and [CII] at different wavelengths for those systems observed in range mode. The photometric observations were used to build complete spectral energy distributions (SEDs) from the optical to the far-IR. High-resolution multi-epoch optical spectra with high signal-to-noise ratios were also analysed to study the multiplicity of the sources and look for further gas (accreting) and outflow indicators. Results: We detect four out of fifteen sources observed at 70 μm, four out of six at 100 μm, and six out of sixteen at 160 μm. Only one system shows [OI] emission at 63 μm, namely RECX 15 or J0843.3-7905. None of them shows far-IR line emission at any other wavelength. The [OI] emission toward RECX 15 points to the presence of an outflow; however, the emission is not extended. We study Hα emission among η Cha members and conclude that RECX 4, 5, 9, 11, and 15 are actively accreting in at least one epoch. Conclusions: The SEDs of the discs in η Cha show a variety of shapes, from those in Taurus and in Upper Scorpius to sources showing excess over the Taurus median SED. Furthermore, the SEDs of RECX 3 and RECX 4 are typical of debris discs. The detection fraction for [OI] in η Cha is lower than younger regions like Taurus and Cha II, indicative of an evolutionary trend. The lack of [OI] emission, together with the intermediate values of the IR excess, can be explained by long-lived discs with a flattened geometry or by flared discs with a low UV flux, or by a combination of the two scenarios. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

THE FATE OF FALLBACK MATTER AROUND NEWLY BORN COMPACT OBJECTS

The presence of fallback disks around young neutron stars has been invoked over the years to explain a large variety of phenomena. Here we perform a numerical investigation of the formation of such disks during a supernova explosion, considering both neutron star (NS) and black hole (BH) remnants. Using the public code MESA, we compute the angular momentum distribution of the pre-supernova material, for stars with initial masses M in the range 13 - 40 Msun, initial surface rotational velocities vsurf between 25% and 75% of the critical velocity, and for metallicities Z of 1%, 10% and 100% of the solar value. These pre SN models are exploded with energies E varying between 10^{50} - 3x10^{52} ergs, and the amount of fallback material is computed. We find that, if magnetic torques play an important role in angular momentum transport, then fallback disks around NSs, even for low-metallicity main sequence stars, are not an outcome of SN explosions. Formation of such disks around young NSs can only happen under the condition of negligible magnetic torques and a fine-tuned explosion energy. For those stars which leave behind BH remnants, disk formation is ubiquitous if magnetic fields do not play a strong role; however, unlike the NS case, even with strong magnetic coupling in the interior, a disk can form in a large region of the {Z,M,vsurf,E} parameter space. Together with the compact, hyperaccreting fallback disks widely discussed in the literature, we identify regions in the above parameter space which lead to extended, long-lived disks around BHs. We find that the physical conditions in these disks may be conducive to planet formation, hence leading to the possible existence of planets orbiting black holes.

Triaxial cosmological haloes and the disc of satellites

We construct simple triaxial generalisations of Navarro-Frenk-White haloes. The models have elementary gravitational potentials, together with a density that is cusped like 1/r at small radii and falls off like 1/r^3 at large radii. The ellipticity varies with radius in a manner that can be tailored to the user's specification. The closed periodic orbits in the planes perpendicular to the short and long axes of the model are well-described by epicyclic theory, and can be used as building blocks for long-lived discs. As an application, we carry out the simulations of thin discs of satellites in triaxial dark halo potentials. This is motivated by the recent claims of an extended, thin disc of satellites around the M31 galaxy with a vertical rms scatter of ~12 kpc and a radial extent of ~ 300 kpc (Ibata et al. 2013). We show that a thin satellite disc can persist over cosmological times if and only if it lies in the planes perpendicular to the long or short axis of a triaxial halo, or in the equatorial or polar planes of a spheroidal halo. In any other orientation, then the disc thickness doubles on ~5 Gyr timescales and so must have been born with an implausibly small vertical scaleheight.

Relic proto-stellar discs and the origin of luminous circumstellar interaction in core-collapse supernovae

A small fraction of core collapse supernovae (SNe) show evidence that the outgoing blast wave has encountered a substantial mass ~ 1-10 M_sun of circumstellar matter (CSM) at radii ~100-1000 AU, much more than can nominally be explained by pre-explosion stellar winds. In extreme cases this interaction may power the most luminous, optically-energetic SNe yet discovered. Interpretations for the origin of the CSM have thus far centered on explosive eruptions from the star just ~ years to decades prior to the core collapse. Here we consider an alternative possibility that the inferred CSM is a relic disk left over from stellar birth. We investigate this hypothesis by calculating the evolution of proto-stellar disks around massive stars following their early embedded phase using a self-similar accretion model. We identify an initial gravitationally-unstable ("gravito-turbulent") phase, followed by a much longer period of irradiation-supported accretion during which less effective non-gravitational forms of angular momentum transport dominate. Although external influences, such as the presence of a wide binary companion, may preclude disk survival in many systems, we find that massive (~1-10 M_sun) disks can preferentially survive around the most massive stars. Reasons for this perhaps counterintuitive include (1) the shorter stellar lifetimes and (2) large photo-evaporation radii (~ 1000 AU) of very massive stars; (3) suppression of the magneto-rotational instability due to the shielding from external sources of ionization; and (4) relative invulnerability of massive disks to lower mass stellar collisions and luminous blue variable eruptions. Because very luminous SNe are rare, testing the relic disk model requires constraining the presence of long-lived disks around a small fraction of very massive stars.

THE DUSTY CIRCUMSTELLAR DISKS OF B[e] SUPERGIANTS IN THE MAGELLANIC CLOUDS

To better ascertain the nature of the infrared excesses that are characteristic of B[e] supergiants, we obtained Spitzer IRS spectroscopy and IRAC/MIPS imaging for a sample of nine B[e] supergiant stars in the Magellanic Clouds. We find that all nine stars display mid- to far-IR spectral and spatial characteristics indicative of the presence of circumstellar dust disks. Several of the sample B[e] supergiants display crystalline silicate features in their IRS spectra, consistent with grain processing in long-lived (i.e., orbiting) disks. Although it is possible that these disks are primordial in origin, large shell structures (with size scales of tens of parsec) are associated with five of the nine B[e] supergiants, suggesting that mass loss has provided the circumstellar material now orbiting these stars. Hence—via analogy to the class of post-asymptotic giant branch stars with binary companions and dusty, circumbinary disks—we speculate that B[e] supergiant stars may be post-red supergiants in binary systems with orbiting, circumbinary disks that are derived from post-main-sequence mass loss.

Long-lived planetesimal discs

We investigate the survival of planetesimal discs over Gyr timescales, using a unified approach that is applicable to all Keplerian discs of solid bodies -- dust grains, asteroids, planets, etc. Planetesimal discs can be characterized locally by four parameters: surface density, semi-major axis, planetesimal size and planetesimal radial velocity dispersion. Any planetesimal disc must have survived all dynamical processes, including gravitational instability, dynamical chaos, gravitational scattering, physical collisions, and radiation forces, that would lead to significant evolution over its lifetime. These processes lead to a rich set of constraints that strongly restrict the possible properties of long-lived discs. Within this framework, we also discuss the detection of planetesimal discs using radial velocity measurements, transits, microlensing, and the infrared emission from the planetesimals themselves or from dust generated by planetesimal collisions.

MINIMUM MASS SOLAR NEBULAE AND PLANETARY MIGRATION

The Minimum Mass Solar Nebula (MMSN) is a protoplanetary disk that contains the minimum amount of solids necessary to build the planets of the Solar System. Assuming that the giant planets formed in the compact configuration they have at the beginning of the "Nice model", Desch (2007) built a new MMSN. He finds a decretion disk, about ten times denser than the well-known Hayashi MMSN. The disk profile is almost stationary for about ten million years. However, a planet in a protoplanetary disk migrates. In a massive, long-lived disk, this question has to be addressed. With numerical simulations, we show that the four giant planets of the Solar System could not survive in this disk. In particular, Jupiter enters the type III, runaway regime, and falls into the Sun like a stone. Known planet-planet interaction mechanisms to prevent migration, fail in this nebula, in contrast to the Hayashi MMSN. Planetary migration constrains the construction of a MMSN. We show how this should be done self-consistently.

Lithium depletion and the rotational history of exoplanet host stars

Israelian et al. (2004) reported that exoplanet host stars are lithium depleted compared to solar-type stars without detected massive planets, a result recently confirmed by Gonzalez (2008). We investigate whether enhanced lithium depletion in exoplanet host stars may result from their rotational history. We have developed rotational evolution models for slow and fast solar-type rotators from the pre-main sequence (PMS) to the age of the Sun and compare them to the distribution of rotational periods observed for solar-type stars between 1 Myr and 5 Gyr. We show that slow rotators develop a high degree of differential rotation between the radiative core and the convective envelope, while fast rotators evolve with little core-envelope decoupling. We suggest that strong differential rotation at the base of the convective envelope is responsible for enhanced lithium depletion in slow rotators. We conclude that lithium-depleted exoplanet host stars were slow rotators on the zero-age main sequence (ZAMS) and argue that slow rotation results from a long lasting star-disk interaction during the PMS. Altogether, this suggests that long-lived disks (> 5 Myr) may be a necessary condition for massive planet formation/migration.

Mergers of Black Hole–Neutron Star Binaries. I. Methods and First Results

We use a 3D relativistic SPH (smoothed particle hydrodynamics) code to study mergers of black hole-neutron star (BH-NS) binary systems with low mass ratios, adopting q ≡ MNS/MBH 0.1 as a representative case. The outcome of such mergers depends sensitively on both the magnitude of the BH spin and its obliquity (i.e., the inclination of the binary orbit with respect to the equatorial plane of the BH). In particular, only systems with sufficiently high BH spin parameter a and sufficiently low orbital inclinations allow any NS matter to escape or to form a long-lived disk outside the BH horizon after disruption. Mergers of binaries with orbital inclinations above ~60° lead to complete prompt accretion of the entire NS by the BH, even for the case of an extreme Kerr BH. We find that the formation of a significant disk or torus of NS material around the BH always requires a near-maximal BH spin and a low initial inclination of the NS orbit just prior to merger.

Long-lived disc accretion in the η Chamaeleontis pre-main sequence star cluster

High-resolution spectroscopic study of late-type members of the ≈9-L39-old η Chamaeleontis star cluster shows that four stars, RECX 5, 9, 11 and ECHA J0843.3–7905, have broad Hα profiles indicative of ballistic accretion of material from circumstellar discs first identified by virtue of their infrared (IR) excess emission. Quantitative analysis of the profiles finds accretion in η Cha stars at rates comparable to that derived by Muzerolle et al. for members of the similarly aged TW Hydrae Association (TWA); rates 1–3 orders of magnitude lower than in younger classical T Tauri (CTT) stars. Together these studies indicate that the fraction of long-lived inner discs can be significantly higher than that inferred from study of younger pre-main sequence (PMS) populations, which suggest a disc lifetime of <6 Myr. The detection of long-lived discs may have implications for the formation of planetary systems. If slow accretion processes are the dominant formation mechanism for Jovian planets then long-lived discs may be ideal sites to search for evidence for protoplanets.