Residual Disc Research Articles

In Response to Letter Re: Postoperative Longitudinal Outcomes in Patients with Residual Disc Fragments

In Response to Letter Re: Postoperative Longitudinal Outcomes in Patients with Residual Disc Fragments

Re: Baek et al Postoperative Longitudinal Outcomes in Patients with Residual Disc Fragments after Percutaneous Endoscopic Lumbar Discectomy

Re: Baek et al Postoperative Longitudinal Outcomes in Patients with Residual Disc Fragments after Percutaneous Endoscopic Lumbar Discectomy

Sacral Dysmorphism and its Implication on the Size of the Sacroiliac Joint Surface.

This is a retrospective data analysis. The aim of this study was to analyze the prevalence of sacral dysmorphism and its correlation to the size of the sacroiliac joint (SIJ) surface based on computed tomography (CT) scans. Sacroiliac screw fixation is a widely accepted technique for stabilization of posterior pelvic ring injuries. Safe sacral screw placement may be impaired by sacral dysmorphism. The prevalence and impact of sacral dysmorphism on the size of the SIJ surface is unknown. In total, 269 CT scans were evaluated for the presence of the 5 signs of sacral dysmorphism (mammillary bodies, tongue-in-groove, residual upper sacral disk space, colinearity, and dysmorphic sacral neural foramina). The size of the SIJ surface was calculated by measuring the sacral joint line of the SIJ on each axial CT slice. Logistic regression analyses were conducted to reveal sex-related or age-related differences and correlations between the presence of the dysmorphic signs and the size of the SIJ surface. Prevalence rates of the dysmorphic signs ranged from 5% (colinearity) to 70% (residual sacral disk space). Only 15% did not show any sign of sacral dysmorphism. The average size of the SIJ surface was 7.36 cm; it was significantly larger in male (8.46 cm) than in female (6.11 cm) patients (P<0.001). The presence of tongue-in-groove morphology was associated with a significantly larger SIJ surface (P<0.001), the presence of a residual upper sacral disk space with a significantly smaller joint surface (P=0.006). The prevalence of sacral dysmorphism is remarkably high in a normal population and it is questionable if the respective signs should be called dysmorphic after all. The possibility of a smaller joint surface in female patients and patients with a residual upper sacral disk space should be considered in the planning of iliosacral screw placement.

Does Preservation of Ligamentum Flavum in Percutaneous Endoscopic Lumbar Interlaminar Discectomy Improve Clinical Outcomes?

ObjectiveLigamentum flavum (LF) is an important anatomical structure for prevention of postoperative adhesions, but the opening of LF is necessary for percutaneous endoscopic lumbar interlaminar discectomy (PEID). Although the defect in LF is small with conventional PEID, the defect could be minimized with LF splitting technique. The objective of this study was to compare clinical outcomes of PEID with opening of LF versus splitting of LF. MethodsA retrospective study was performed for patients underwent PEID for L5–S1. PEID with the opening of LF (open-group) was performed for 55 patients and with splitting of LF (split-group) was performed for 34 patients. The defect of LF in Open-group was 3–5 mm, but the defect was negligible in split-group because the split LF was reapproximated by its elasticity. Clinical outcomes were evaluated with Korean version of the Oswestry Disability Index (K-ODI) and visual analogue pain scores for back (VASB) and leg (VASL). The changes of clinical outcomes during postoperative 24 months between groups were evaluated with linear mixed-effects model. ResultsThe clinical outcomes were similar between groups for K-ODI (p=0.98), VASB (p=0.52), and VASL (p=0.59). Each outcome demonstrated significant improvement from preoperative baseline throughout the postoperative 24 months (p<0.05). Complications included recurrence in 4 patients and dural tear in 1 in open-group (9.1%), and residual disc herniation in 2 patients and transient weakness in 1 in split-group (8.8%). ConclusionSplitting versus opening LF in PEID may be left to the surgeon’s discretion. The potential risks and benefits of LF handling should be considered when performing this surgical technique in PEID.

Fast, Ultraluminous X-Ray Bursts from Tidal Stripping of White Dwarfs by Intermediate-mass Black Holes

Abstract Two X-ray sources were recently discovered by Irwin et al. that are compact companions to elliptical galaxies showing ultraluminous flares with fast rise (∼minute) and decay (∼hour), and with a peak luminosity ∼1040–41 erg s−1. Together with two other sources found earlier, they constitute a new type of fast transients that cannot be attributed to neutron stars, but might be due to intermediate-mass black holes (IMBHs; ). The flaring behavior is recurrent for at least two sources. If the flare represents a short period of accretion onto an IMBH during the periastron passage of a donor star on an eccentric (i.e., repeating) or parabolic (non-repeating) orbit, we argue that the flare’s rise time corresponds to the duration during which the donor’s tidally stripped mass joins a residual disk at the pericenter. This duration is in turn equal to three other timescales: the duration of stripping, the sound crossing time of the donor, and the circular orbit time at the pericenter radius. Only a white dwarf (WD) can have a sound crossing time as short as one minute. Therefore, the donor must be a WD and it was stripped of ∼10−10 M ⊙ upon each passage at several to tens of Schwarzschild radii from the IMBH. The flux decay corresponds to the viscous drainage of the supplied mass toward the hole. Aided by long-term X-ray monitoring, this type of fast transient would be an ideal target for next-generation gravitational wave detectors.

Transfacet and Transpedicular Posterior Approaches to Thoracic Disc Herniations: Consecutive Case Series of 24 Patients

Various approaches are advocated for symptomatic thoracic disc herniation (TDH). The aim of this series is to demonstrate the feasibility, safety, and results of posterior transfacet or transpedicular approaches for excision of all types of extradural TDH. We report a consecutive series of patients undergoing posterior approach surgery for TDH. Twenty-four patients (17 women, 7 men) underwent surgery at 25 disc levels. Mean age was 56.3 years (range, 23-79 years). A posterior transfacet or transpedicular approach was used. Patients presented with myelopathy (n= 21, 88%), radiculopathy (n= 8, 33%), sphincter dysfunction (n= 16, 67%), and axial back pain (n= 10, 43%). Preoperative imaging revealed 7 (30%) central, 14 (61%) calcified, and 10 (43%) large disc herniations. The mean follow-up period was 6.0 months (range, 2-36 months). Eighteen patients underwent unilateral approach surgery (5 transfacet and 13 transfacet plus transpedicular), and 7 patients required bilateral approach laminectomy for unilateral (n= 4) or bilateral (n= 3) discectomy. One patient required unplanned reoperation for resection of residual disc. Average operative time was 95 minutes (range, 40-175 minutes). Mean hospital stay was 4.9 days (range, 2-35 days). There were no major complications. Postoperative Frankel scores were maintained or improved in all patients at last review. TDH including large central calcified discs can be safely removed through posterior transfacet or transpedicular approaches with reduced morbidity in comparison with more invasive anterior approaches. Careful microsurgical technique and use of specialized instruments are important for successful excision of TDH from a posterior approach.

Development of a Virtual Reality Preoperative Planning System for Postlateral Endoscopic Lumbar Discectomy Surgery and Its Clinical Application

Percutaneous endoscopic lumbar discectomy is an effective way to treat lumbar disc herniation. Traditional preoperative planning based on a 2-dimensional method by magnetic resonance/computed tomography may cause inaccuracy of puncture during surgery. We used virtual reality to stimulate a surgery environment and measured relevant 3-dimensional data. We then explored its applicability for increasing puncture accuracy during actual surgeries. A prospective randomized trial of lumbar disc herniation was conducted. Both conventional and virtual reality methods were used for preoperative planning and relevant data (planned puncture point and entry angle) were measured. Data were used during surgery and adjusted to complete the operation. The final entry point and entry angle were recorded and compared with relevant planned data statistically. Fluoroscopic times and location time also were included to access the puncture accuracy during surgery. Thirty cases were included in our study. Both groups achieved good results after surgery, except for 1 case of postoperative dysesthesia in the traditional planning group and 1 case of residual disc in the virtual reality group. The use of virtual reality can predict a surgery-related angle and distance accurately except for depth. Compared with the traditional planning group, the fluoroscopic time (13.18 ± 4.191 vs. 32.00 ± 4.52) and location time (17.91 ± 4.74 vs. 33.22 ± 3.90) were statistically different, which indicates that this method can increase puncture accuracy. A virtual reality planning system is an accurate preoperative planning method that can significantly improve the puncture accuracy of percutaneous endoscopic lumbar discectomy and reduce fluoroscopic and location times.

Postoperative Longitudinal Outcomes in Patients with Residual Disc Fragments after Percutaneous Endoscopic Lumbar Discectomy

Background: Residual disc fragments are observed on immediate postoperative magnetic resonance imaging (MRI) in 2.8–15% of patients after percutaneous endoscopic lumbar discectomy (PELD). Considering the known postoperative longitudinal outcomes in patients with residual disc tissue, a ‘watchful waiting’ strategy may be preferable to immediate re-operation in patients with asymptomatic residual disc material. Objectives: The aim of the present study was to compare the longitudinal clinical outcomes between PELD patients in whom the complete removal of disc fragments was achieved (complete group) and those in whom residual disc fragments were observed on postoperative MRI (residual group). Study Design: Retrospective nested case-control study. Methods: A total of 225 patients were included (complete group, n=187 and residual group, n=38). Clinical assessments were performed using the visual analog pain score for the leg (VAS-L, x/10) and back (VAS-B, x/10) and the Korean version of the Oswestry Disability Index (K-ODI, x/45). A linear mixed-effects model was used to analyze changes during the first 24 postoperative months. Results: One month after surgery, significant improvements in the VAS-L, VAS-B and K-ODI values were observed and were maintained during the first 24 postoperative months. No differences in these changes were noted between the groups. Early reoperation (during the first 3 postoperative months) was performed in 3 patients in the residual group (7.9%) and in 4 patients in the complete group (2.1%) (P = 0.10). Limitations: First, the study design was retrospective. Moreover, the number of patients was relatively small and therefore insufficient to achieve robust statistical power. Second, we did not explore the radiological outcomes in patients with asymptomatic residual disc material because follow-up MRI was only obtained to document symptom recurrence. Conclusion: When residual disc tissue is observed in asymptomatic patients, a ‘watchful waiting’ strategy may be preferable to immediate re-operation. However, an increased early re-operation rate is expected for patients with residual disc tissue. Key words: Discectomy, endoscopes, longitudinal studies, patient-reported outcome, percutaneous, reoperation, spine, residual disc

Microendoscopic discectomy for lumbar disc herniations.

Introduction:Lumbar disc herniation is one of the main causes of discogenic low back pain and reported to affect 60%–80% of people during their lifetime. The two main surgical modalities for intervertebral disc surgery are standard open discectomy and minimally invasive discectomy which include percutaneous endoscopic lumbar discectomy and microendoscopic discectomy (MED). We report our experience with the same technique of MED to evaluate the efficacy of MED for lumbar disc pathology.Aims and Objectives:The aims and objectives were to study the efficacy, advantages, and associated limitations and complications of MED in lumbar disc herniations.Materials and Methods:This study was carried out on 300 patients who had single-level herniated disc. The procedure was done by Microscopic Endoscopic Tubular Retraction System. Preoperative assessment of Visual Analog Scale (VAS) and modified Suezawa and Schreiber (MSS) clinical scoring system was documented 1 day prior to surgery. Postoperative results were determined to be excellent, good, fair, or poor according to MacNab criteria and also evaluated by MSS clinical scoring system on postoperative day 7 and after 6 months.Results:A total of 187 patients were males and 113 patients were females and a majority of patients were in the age group of 31–40 years. A total of 192 patients had disc herniations at L4–L5 level. The mean operative time was 82 min and the mean hospital stay was 5.3 days. Eighteen cases (6%) developed postoperative complications including discitis, dysesthesia, recurrent prolapsed intervertebral disc, residual disc, dural tear, and nerve root injury. Mean preoperative VAS score was 8.7 and the mean postoperative VAS scores at postoperative day 7 and at 6 months were 2.25 and 1.12, respectively. The mean preoperative MSS score was 3.27 and the MSS scores at postoperative day 7 and at 6 months were 7.42 and 8.2, respectively. The overall successful outcome of the endoscopic discectomy after 6-month follow-up on the basis of VAS improvement percentage was 87.6%, MSS scoring percentage was 91.6%, and MacNab scoring percentage was 92.67%.Conclusion:MED is a safe and effective technique. It offers decreased blood loss, shorter operative time, shorter in-hospital stay, decreased need for pain medication, decreased rate of infection, and a shorter return to work time. Limitations of this technique include a learning curve which is related to surgery time, complications, conversion to open procedures, and recurrent disc herniation.

Peri-operative Radiological Factors Predicting Outcome of Discogenic Cauda Equina Syndrome

Background Data: Acute cauda equina syndrome (CES) is a rare disorder considered as a neurosurgical emergency. It can be devastating in some circumstances. Outcome can be predicted by certain preoperative radiological and clinical items.Purpose: To evaluate and analyse the effect of radiological criteria either pre- or postoperative on the final outcome of acute cauda equina syndrome.Study Design: A prospective clinical case study on 27 patients who were presented by manifestations of CES from May 2014 to September 2017.Patients and Methods: Between May 2014 to May 2017 a total of 27 consecutive patients with discogenic acute cauda equina syndrome underwent decompressive surgery (laminectomy and discectomy) with a follow-up at 6 and 12 months postoperative. Preoperative and postoperative clinical assessments were done by using Visual Analogue Scale (VAS) for analysing pain and full neurological examination. Surgical outcome was assessed using Oswestry Disability Index (ODI) and Japanese Orthopedic Association (JOA) scale. Recovery rate (RR) was calculated in final follow up for each patient. In all cases, preoperative and at the last follow-up control a neuro-radiographic MRI assessment was done. Level of affection, size and direction of prolapsed disc, presence of lumbar canal stenosis, degree of decompression (laminectomy), and the presence of disc residual disc, were all thoroughly analyzed with evaluating the correlation with clinical outcome.Results: At final follow-up visit, we found that radiological factors had significant effect on the overall clinical outcome. L5-S1 disc level had the most favourable outcome when compared to other affected level. A statically significant correlation was found between degree of decompression and overall clinical improvement (P=0.001). Residual disc fragments had a direct relation to incomplete recovery from preoperative symptoms although statistically insignificant (P=0.93).Conclusion: Full and thorough study of the pre- and postoperative radiological findings of the patients presented with discogenic cauda equina syndrome has an important prognostic value that can give a prediction for the surgical outcome and the overall clinical improvement. The degree of decompression and presence of canal stenosis was the most important outcome predictor factors. (2017ESJ148)

Clearing Residual Planetesimals by Sweeping Secular Resonances in Transitional Disks: A Lone-planet Scenario for the Wide Gaps in Debris Disks around Vega and Fomalhaut

Abstract Extended gaps in the debris disks of both Vega and Fomalhaut have been observed. These structures have been attributed to tidal perturbations by multiple super-Jupiter gas giant planets. Within the current observational limits, however, no such massive planets have been detected. Here we propose a less stringent “lone-planet” scenario to account for the observed structure with a single eccentric gas giant and suggest that clearing of these wide gaps is induced by its sweeping secular resonance. With a series of numerical simulations, we show that the gravitational potential of the natal disk induces the planet to precess. At the locations where its precession frequency matches the precession frequency the planet imposes on the residual planetesimals, their eccentricity is excited by its resonant perturbation. Due to the hydrodynamic drag by the residual disk gas, the planetesimals undergo orbital decay as their excited eccentricities are effectively damped. During the depletion of the disk gas, the planet’s secular resonance propagates inward and clears a wide gap over an extended region of the disk. Although some residual intermediate-size planetesimals may remain in the gap, their surface density is too low to either produce super-Earths or lead to sufficiently frequent disruptive collisions to generate any observable dusty signatures. The main advantage of this lone-planet sweeping-secular-resonance model over the previous multiple gas giant tidal truncation scenario is the relaxed requirement on the number of gas giants. The observationally inferred upper mass limit can also be satisfied provided the hypothetical planet has a significant eccentricity. A significant fraction of solar or more massive stars bear gas giant planets with significant eccentricities. If these planets acquired their present-day kinematic properties prior to the depletion of their natal disks, their sweeping secular resonance would effectively impede the retention of neighboring planets and planetesimals over a wide range of orbital semimajor axes.

P171 - Complete Debridement for Treatment of Thoracolumbar Spinal Tuberculosis: A Clinical Curative Effect Observation

BACKGROUND CONTEXT: Traditional focal debridement involves clearing of cold abscesses, caseous necrosis, residual intervertebral disc tissue, sinus tracts, bony sequestration and inflammatory granulation. Reports have demonstrated that approximately 13% to 26% of patients were not better or relapsed after traditional focal debridement; these patients required a second surgery or prolonged antituberculous therapy. The presence of retained and diseased focal tissues requiring removal remains poorly understood. The contents of these retained tuberculous foci improvement of surgical strategies, and improvement in spinal tuberculosis success rate are key subjects for discussion.

The mass of the black hole in 1A 0620–00, revisiting the ellipsoidal light curve modelling

The mass distribution of stellar mass black holes can provide important clues to supernova modeling, but observationally it is still ill constrained. Therefore it is of importance to make black hole mass measurements as accurate as possible. The X-ray transient 1A 0620-00 is well studied, with a published black hole mass of $6.61\pm0.25\,$M$_{\odot}$, based on an orbital inclination $i$ of $51.0\pm0.9$ degrees. This was obtained by Cantrell et al. (2010), as an average of independent fits to $V$-, $I$- and $H$-band light curves. In this work we perform an independent check on the value of $i$ by re-analyzing existing YALO/SMARTS $V$-, $I$- and $H$-band photometry, using different modeling software and fitting strategy. Performing a fit to the three light curves simultaneously, we obtain a value for $i$ of $54.1\pm1.1$ degrees, resulting in a black hole mass of $5.86\pm0.24\,$M$_{\odot}$. Applying the same model to the light curves individually, we obtain $58.2\pm1.9$, $53.6\pm1.6$ and $50.5\pm2.2$ degrees for $V$-, $I$- and $H$-band, respectively, where the differences in best-fitting $i$ are caused by the contribution of the residual accretion disc light in the three different bands. We conclude that the mass determination of this black hole may still be subject to systematic effects exceeding the statistical uncertainty. Obtaining more accurate masses would be greatly helped by continuous phase-resolved spectroscopic observations simultaneous with photometry.

Anterior discectomy could still be an alternative to corpectomy in highly migrated cervical disc herniation

Objective: For cases of cervical disc herniation, highly migrated cervical disc (HMCD) is clinically rare and usually treated with anterior cervical corpectomy and fusion (ACCF). This study aims to analyze the feasibility of anterior cervical discectomy and fusion (ACDF) for the patients with HMCD.Method: Clinical data of 32 patients with HMCD treated with ACDF or ACCF were retrospectively reviewed. Migration distances of the disc prolapses were measured. The mJOA score was used to evaluate surgical effect.Results: ACDF was successful in 27 patients while ACCF was used for the remaining 5 because of epidural disc prolapse adhesion or unreachable migrated fragments. Complete spinal cord decompression without residual disc fragments was observed in postoperative MRI of all cases. The mean migration distance of the disc prolapses in ACDF group was 7.3 mm, comparing to 11.4 mm in ACCF group. No disc prolapse in ACDF group exceeded the axial length of the vertebral bodies while three of five in ACCF group did. Preoperative mean mJOA scores in ACDF group and ACCF group were 8.20 ± 2.75 and 6.10 ± 2.15, respectively. Postoperative mean mJOA scores in those two groups were significantly improved to 14.70 ± 1.55 (p < .001) and 12.80 ± 1.72 (p < .001), with an improvement rate of 72.80 ± 4.76% and 62.90 ± 9.46%, respectively.Conclusion: ACDF is feasible for patients with HMCD except for cases of epidural disc prolapse adhesion or huge disc prolapse which migrates over the axial length of the vertebral body. Clinical symptoms can be significantly improved with few serious complications in those patients including ones underwent alternative ACCF due to a failed ACDF.

Giant thoracic discs: treatment, outcome, and follow-up of 33 patients in a single centre.

To help guide treatment strategies and create insight into functional outcomes in patients with Giant herniated thoracic discs (GHTD), which are defined as occupying more than 40% of spinal canal. Authors did a retrospective analysis of prospective cohort of 33 cases of GHTD, using clinical letters, notes, and telephonic questionnaires to determine their pre and postoperative functional status, surgical details, and complication rates. 16 males and 17 females operated between 2006 and 2014 were included in the study. A total of 23 patients underwent thoracotomy, 9 costotransversectomy, and 2 transpedicular approaches for excision of thoracic discs. Neuromonitoring was used in seven patients. Frankel grade improved by 1 in 13 patients and by 2 in 1 patient. One patient of T11/12 GHTD with neurogenic claudication recovered completely, taking the overall improvement rate to 45.5%. It remained static in 15 patients (45.5%) and deteriorated by 1 in 3 patients (9%). By mJOA scoring too, the outcome was favorable in majority (84.4%) of patients. There were three intraoperative complications (9%), which included two incidental durotomies and one massive blood loss. Late postoperative complications were seen in 12(39%) patients. They included intercostal neuralgia, mechanical pain around costotransversectomy, progressive thoracic paraplegia due to spinal cord herniation and residual disc fragments, reactive pleural effusion, CSF fistula induced pleural effusion, and infective discitis. Surgery for giant herniated thoracic disc has favorable outcome in majority (91%) of patients. However, significant approach-related complications are to be anticipated in patients undergoing thoracotomies, most of them being manageable. Author recommends costotransversectomy, only in paracentral discs with smaller percentage canal stenosis.

The cooling, mass and radius of the neutron star in EXO 0748−676 in quiescence with XMM–Newton

We analyse four XMM-Newton observations of the neutron-star low-mass X-ray binary EXO 0748$-$676 in quiescence. We fit the spectra with an absorbed neutron-star atmosphere model, without the need for a high-energy (power-law) component; with a 95 per cent confidence the power-law contributes less than 1 per cent to the total flux of the source in $0.5-10.0$ keV. The fits show significant residuals at around 0.5 keV which can be explained by either a hot gas component around the neutron star or a moderately broad emission line from a residual accretion disc. The temperature of the neutron-star has decreased significantly compared to the previous observation, from 124 eV to 105 eV, with the cooling curve being consistent with either an exponential decay plus a constant or a (broken) power law. The best-fitting neutron-star mass and radius can be better constrained if we extend the fits down to the lowest possible energy available. For an assumed distance of 7.1 kpc, the best-fitting neutron-star mass and radius are $2.00_{-0.24}^{+0.07}~M_\odot$ and $11.3_{-1.0}^{+1.3}$ km if we fit the spectrum over the $0.3-10$ keV range, but $1.50_{-1.0}^{+0.4}~M_\odot$ and $12.2_{-3.6}^{+0.8}$ km if we restrict the fits to the $0.5-10$ keV range. We finally discuss the effect of the assumed distance to the source upon the best-fitting neutron-star mass and radius. As systematic uncertainties in the deduced mass and radius depending on the distance are much larger than the statistical errors, it would be disingenuous to take these results at face value.

Residual Gas and Dust around Transition Objects and Weak T Tauri Stars*

Abstract Residual gas in disks around young stars can spin down stars, circularize the orbits of terrestrial planets, and whisk away the dusty debris that is expected to serve as a signpost of terrestrial planet formation. We have carried out a sensitive search for residual gas and dust in the terrestrial planet region surrounding young stars ranging in age from a few to ∼10 Myr. Using high-resolution 4.7 μm spectra of transition objects (TOs) and weak T Tauri stars, we searched for weak continuum excesses and CO fundamental emission, after making a careful correction for the stellar contribution to the observed spectrum. We find that the CO emission from TOs is weaker and located farther from the star than CO emission from nontransition T Tauri stars with similar stellar accretion rates. The difference is possibly the result of chemical and/or dynamical effects (i.e., a low CO abundance or close-in low-mass planets). The weak T Tauri stars show no CO fundamental emission down to low flux levels (5 × 10−20 to 10−18 W m−2). We illustrate how our results can be used to constrain the residual disk gas content in these systems and discuss their potential implications for star and planet formation.

A second decoupling between merging binary black holes and the inner disc – impact on the electromagnetic counterpart

Abstract The coalescence of two supermassive black holes (SMBHs) produces powerful gravitational wave radiation and, if gas is present in the vicinity, also an electromagnetic (EM) counterpart. In the standard picture, an EM outburst will be produced when the binary ‘decouples’ from the circum-binary disc and starts ‘squeezing’ the disc inside the secondary orbit, resulting in its quick accretion on to the primary black hole. Here, we use analytical arguments and numerical simulations to show that the disc within about 20 RS of an SMBH survives the merger without being depleted. The reason is a ‘second decoupling’: the inner disc thickens due to tidal heating and inefficient cooling, effectively decoupling from the interaction of the binary. We show that this second decoupling quenches the heating sources in the disc ${\cal O}(10^2)$ d before coalescence. This will render the peak UV/X-ray luminosity significantly weaker than previously thought. After the merger, the residual disc cools down and expands, merging with the outer disc rather than being completely accreted. This results in continuous EM emission, hindering the detection of the cut-off and re-brightening proposed in earlier studies.

ROCKY PLANET FORMATION: QUICK AND NEAT

ABSTRACT We reconsider the commonly held assumption that warm debris disks are tracers of terrestrial planet formation. The high occurrence rate inferred for Earth-mass planets around mature solar-type stars based on exoplanet surveys (∼20%) stands in stark contrast to the low incidence rate (≤2%–3%) of warm dusty debris around solar-type stars during the expected epoch of terrestrial planet assembly (∼10 Myr). If Earth-mass planets at au distances are a common outcome of the planet formation process, this discrepancy suggests that rocky planet formation occurs more quickly and/or is much neater than traditionally believed, leaving behind little in the way of a dust signature. Alternatively, the incidence rate of terrestrial planets has been overestimated, or some previously unrecognized physical mechanism removes warm dust efficiently from the terrestrial planet region. A promising removal mechanism is gas drag in a residual gaseous disk with a surface density ≳10−5 of the minimum-mass solar nebula.

M STARS IN THE TW HYA ASSOCIATION: STELLAR X-RAYS AND DISK DISSIPATION

ABSTRACT To investigate the potential connection between the intense X-ray emission from young low-mass stars and the lifetimes of their circumstellar planet-forming disks, we have compiled the X-ray luminosities (L X ) of M stars in the ∼8 Myr old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing with decreasing T eff for TWA M stars, wherein the earliest-type (M0–M2) stars cluster near and then decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.