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The young planetary system surrounding the star β Pictoris harbours active minor bodies1-6. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System7. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies8-11, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, whichmay be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.
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From 2003 to 2011, a total of 1,106 spectra of b Pictoris have been obtained using theHARPS(High AccuracyRadial velocity Planet Searcher) spectrograph. Observations of the calcium (Ca II) doublet.the Ca II K-line at 3,933.66A¢a and the Ca II H-line at 3,968.47A¢a .show a large number of variable absorption features (Fig. 1) varying on timescales of one to six hours. These features, simultaneously detected in bothCa II K and Ca II H lines, are interpreted as exocomets transiting in front of the stellar disk7.11. Since the b Pic Ca II spectrumis typically observed to be stable on 30-min timescales, we averaged together spectra in distinct 10-min time intervals to limit any possible spectral variability. This results in a total of 357 spectra with signal-to-noise ratio greater than 80. Tocharacterize the profile of these transient absorptionlines,wedivided each of the 357 averaged spectra by a reference spectrum of b Pictoris (Extended Data Figs 1 and 2) assumed to be free of the absorption signatures of transiting exocomets.
Given theHARPS resolutionand sensitivity, each bPictoris spectrum shows an average of about...