Mass extinctions and the sun's encounters with spiral arms

Abstract

The terrestrial fossil record shows that the exponential rise in biodiversity since the Precambrian period has been punctuated by large extinctions, at intervals of 40 to 140 Myr. These mass extinctions represent extremes over a background of smaller events and the natural process of species extinction. We point out that the non-terrestrial phenomena proposed to explain these events, such as boloidal impacts (a candidate for the end-Cretaceous extinction), and nearby supernovae, are collectively far more effective during the solar system's traversal of spiral arms. Using the best available data on the location and kinematics of the Galactic spiral structure (including distance scale and kinematic uncertainties), we present evidence that arm crossings provide a viable explanation for the timing of the large extinctions.The literature is replete with suggestions of non-terrestrial phenomena as the candidate causes for large scale extinctions. The most frequently invoked are supernovae and boloidal impacts (e.g. comets from the Oort Cloud), the latter a strong candidate for the K/T extinction ever since the discovery of the Iridium anomaly in the K/T boundary clay (Alvarez et al. 1990). Certainly the most violent events in the solar neighborhood during geologic history would have been supernovae (barring the possibility of a nearby γ-ray burst, which is far less likely; Thorsett 1995); that the structure of the very local interstellar medium is considered to be the result of a supernova, possibly related to the Geminga pulsar (at 150 pc) (Bignami & Caraveo 1996), is an impressive reminder of their potential impact. Supernovae and young supernova remnants, especially those occurring at distances < ∼ 10 pc (Ruderman 1974), can result in biospheric imbalance through a variety of processes, including ozone depletion by enhanced ionizing radiation and cosmic rays (Ellis & Schramm 1995, Koyama et al. 1995, the absorption of visible light by the formation of NO 2 (Crutzen & Brühl 1996), and in rare cases the direct deposition of supernova debris.Tidal and collisional encounters with intermediate-sized gas or dust clouds might focus cometary activity (∼ 10 9 comets) to the inner solar system, by scattering of Oort Cloud member bodies, a mechanism proposed to explain the K/T boundary event. In addition, the passage of the Sun through a cloud of density n > ∼ 10 4 cm −3 could raise the solar luminosity significantly, through Bondi accretion, as well as raise the opacity of Earth's atmosphere, directly affecting the insolation on Earth (McCrea 1975). While the recent association of the Chicxulub crater with the K/T boundary lends credence to the boloidal impact model, the large concentration of Ir deposited at the boundary may indicate that accretion also played an important role (Yabushita & Allen 1997).The proposed mechanisms constitute a set of plausible external agents for any one extinction, yet do not of themselves suggest any explanation for the timing of the mass extinctions, or for the large variation in seve...

Unveiling the nature ofINTEGRALobjects through optical spectroscopy

“…This color excess is substantially lower than the Galactic value of E(B−V) = 3.87 (Schlegel et al 1998). This supports a location for HD 100199 in the near side of the Carina arm (see, e.g., Leitch & Vasisht 1998).…”

“…This distance is compatible with HD 306414 being located in the far end of the Carina arm (see, e.g., Leitch & Vasisht 1998) and implies, if this star is indeed the optical counterpart of IGR J11215−5952, a 20-60 keV peak luminosity of ∼4 × 10 36 erg s −1 . This again compares well with the outburst peak luminosities of SFXTs (Sguera et al 2005;Smith et al 2005).…”

Unveiling the nature ofINTEGRALobjects through optical spectroscopy

“…For Swift J0208.4−7428, the use of our spectroscopy with the available optical photometry (Demers & Irwin 1991) allowed us to state that this is a B-type star and thus improve the preliminary classification given to the putative optical counterpart of this source by McBride et al (2010). The two remaining cases (IGR J08262−3736 and IGR J16327−4940) are instead broadly classified as OB stars due to the strength of their H α emission lines, which is much larger than the typical values seen in blue supergiants (Leitherer 1988); the Galactic Arm model of Leitch & Vasisht (1998) is then used to infer a likely distance of these stars and hence their luminosity class. The lack of further detailed photometric optical information and of higher-resolution spectroscopy does not allow us to refine our classification for the putative counterparts of these two INTEGRAL sources.…”

Unveiling the nature of INTEGRAL objects through optical spectroscopy

“…For IGR J22534+6243, we exclude the possibility that its companion is of luminosity class I because it would lie outside the Galactic disk (>8 kpc from Earth) according to the map of Leicht & Vasisht (1998). Likewise, the presence of substantial reddening towards this source would place the source in or beyond the Perseus Arm of the Galaxy, suggesting a distance >3 kpc.…”