The Age of the Hd 15407 System and the Epoch of Final Catastrophic Mass Accretion Onto Terrestrial Planets Around Sun-Like Stars

Abstract

From optical spectroscopic measurements we determine that the HD 15407 binary system is ∼80 Myr old. The primary, HD 15407A (spectral type F5 V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. Synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly Solar mass this warm dust phenomenon occurs at ages between 30 and 100 Myr. In contrast, for stars of a few Solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30 Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.-2architectures form around other stars. The study of extra-solar terrestrial planet formation must currently be carried out through indirect searches for the dusty aftermath of formation events. Since the dust is located in the terrestrial planet zone, it will have temperatures of ∼300 K and will emit most of its thermal radiation at ∼10 µm. Stars actively undergoing terrestrial planet formation will have substantial amounts of warm dust orbiting in their terrestrial planet zones and will thus show mid-infrared emission in excess of what one would expect from the star alone. Properly establishing the age of such mid-infrared excess systems is necessary for comparison with expectations from planet formation theories and simulations.We were alerted to excess mid-infrared emission around the star HD 15407A through a paper entitled "Detection of silica dust in the debris disk around HD 15407" presented at an October 2009 Spitzer meeting by H. Fujiwara (E. Becklin, personal communication 2009). At this meeting, the age for HD 15407A was suggested to be ∼2 Gyr. We felt this age could be in error, and initiated our own follow-up observations to independently constrain HD 15407A's age. Here we report these observations, our revised value for the age of the HD 15407 binary system, and a discussion of terrestrial planet zone collisions around adolescent Solar mass stars. ObservationsBoth components of the HD 15407 system were observed in cloudy conditions at Mauna Kea Observatory with the Keck I telescope and HIRES echelle spectrometer (Vogt et al. 1994). With the HIRES red collimator wavelengths between 4680 and 8700Å were covered at a resolution of 50,000 (as determined from the FWHM of single ThAr arclines). The final signal-to-noise ratio per pixel, measured at 6700Å, is 350 for HD 15407A and 200 for HD 15407B. Spectral data were reduced and extracted using both the MAKEE software package and standard IRAF tasks and echelle reduction procedures.

Circumstellar debris and pollution at white dwarf stars

“…In fact, the emission observed at GD 362 is among the strongest silicate features seen towards any mature star (compare e.g. Meng et al 2015;Melis et al 2010b). Second, there is a notable lack of relatively cool dust, unlike the bulk of all debris disks orbiting main-sequence stars that exhibit infrared excess only at 24 or 70 µm MIPS observations Wyatt, 2008).…”

Insights into Planet Formation from Debris Disks

“…Early searches for 12 µm emission from dust around nearby stars with IRAS were not particularly successful (Aumann and Probst 1991), though several good candidates were rediscovered in the last decade and followed up with higher resolution instruments to confirm the emission was associated with the star and to characterise its spectrum (e.g., Rhee et al 2008;Melis et al 2010). These hot excesses were picked out on a case-by-case basis, and interpreted again within the context of terrestrial planet formation, based on the fact that the stars with bright emission were predominantly found around < 100 Myr stars.…”

“…There is also growing evidence for giant impacts having occurred recently in some extrasolar planetary systems, from observations of what could be impact-generated debris (Song et al 2005;Rhee et al 2008). Stars with anomalously high levels of dust in their regions that would be analogous to the location of terrestrial planets in the inner Solar System are often interpreted within the context of terrestrial planet formation (e.g., Lisse et al 2008;Melis et al 2010), particularly if they are at an age at which planet formation models predict such giant impacts to be taking place (i.e., 100 Myr). Clearly, studying this debris and how frequently it is detected around nearby stars has the potential to inform about terrestrial planet formation processes more generally, as well as the fraction of stars that host such planets (Jackson and Wyatt 2012).…”

Transient dust in warm debris disks

“…HD 15407 is a binary system with a 21.21 separation. The dust is located around the primary HD 15407 A (F5) and the secondary HD 15407 B (K2, Melis et al 2010) does not appear in the IRS slit, therefore no correction to the spectrum has to be performed for this object.…”

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