[{"id":73,"title":"Extragalactic radio sources as lighthouses for SETI","sha":"be48a68df1c2b616f413017a9138c386","user_id":62,"state":"published","body":"The search for extraterrestrial intelligence (SETI) is one of the most ambitious scientific endeavors. It aims at detecting signals produced intentionally or unintentionally by extraterrestrial civilizations ([Siemion et al., 2013](http://arxiv.org/abs/1412.4867)). The Galaxy, however, is vast and finding suitable telescope targets for SETI is a critical task ([Turnbull \u0026 Tarter, 2003](http://iopscience.iop.org/0067-0049/145/1/181)).\r\n\r\nWe examine a strategy for selecting target areas for (active and passive) SETI aimed at improving chances for success.  The aim is to search for and/or send electromagnetic signals towards and/or from notable extragalactic radio sources.  Assuming that astronomy-wise civilizations are also involved in frequent multi-wavelength monitoring of conspicuous radio sources, the probability for interstellar observers to detect each other when observing in the same direction is increased. A similar idea was proposed by [Lemarchand (1995)](http://link.springer.com/article/10.1007/BF00982337) using Supernova 1987A.  However, radio sources are constant \"lighthouses\" and exist all across the sky.\r\n\r\nFor illustration purposes, we calculate that a signal emitted in a ~1 degree beam in the opposite direction of Centaurus A, could reach ~200 stars, mostly M-dwarfs, at distances \u003c600pc (stellar density model by [Gould et al., 1996](http://iopscience.iop.org/0004-637X/482/2/913)). Conversely if we search for ETI signals around Centaurus A, using the Allen-array (PFOV ~3 degrees, [Deboer et al., 2006](http://link.springer.com/article/10.1007/s10686-005-4193-3)), ~200 stars at distances \u003c300pc will be included in the field and the chance of detecting a message from a civilization using this strategy, is large.","created_at":"2015-02-20T00:46:47.737Z","updated_at":"2018-02-18T07:02:57.524Z","zenodo_id":23173,"doi":"http://dx.doi.org/10.5281/zenodo.15539","subject":null,"tags":["seti","astronomy","astrobiology"],"vote_count":8,"view_count":1,"score":8.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":187,"title":"Supercritical Exoplanetary Atmospheres as Potential Habitats for Extraterrestrial Aeroplankton","sha":"e464b97c0ead0438ccb94a7abe9dbc85","user_id":null,"state":"published","body":"An increasingly familiar class of exoplanets amongst those being continually discovered is the \"Super-Earth\" ([Valencia+2007](http://iopscience.iop.org/0004-637X/656/1/545/)), with a mass intermediate between Earth and Neptune. These have been found inside habitable zones ([Udry+2007](http://www.aanda.org/articles/aa/abs/2007/27/aa7612-07/aa7612-07.html)) and likely have dense atmospheres ([Lopez+2014](http://iopscience.iop.org/0004-637X/792/1/1), [Lammer+2012](http://mnras.oxfordjournals.org/content/430/2/1247)). With sufficient atmospheric pressure and temperature, these atmospheres are likely to enter a supercritical fluid phase with properties intermediate between gas and liquid. Supercritical fluid phases are typically considered for gas giant interiors ([Trachenko+2014](http://journals.aps.org/pre/abstract/10.1103/PhysRevE.89.032126)) or water oceans ([Elkins-Tanton+2008](http://iopscience.iop.org/0004-637X/685/2/1237)), though any gas or liquid can form such a phase. For example, an Earth-like atmospheric composition will enter a supercritical phase at T \u003e 132.6 K and p \u003e 37.21 atm. This pressure is significantly lower than the ≳300 atm ([Koschinsky+2008](http://geology.geoscienceworld.org/content/36/8/615)) experienced by life on the abyssal plain of Earth's oceans ([Rona+1986](http://www.nature.com/nature/journal/v321/n6065/abs/321033a0.html)). It is therefore possible for Earth-like life to tolerate the pressure-temperature conditions of a supercritical atmosphere. \r\n\r\nIn Earth's atmosphere, microbes have been found airborne as \"aeroplankton\" ([Sharma+2007](http://onlinelibrary.wiley.com/doi/10.1111/j.1529-8817.2007.00373.x/abstract)) and living microbes have been found at high altitudes ([Sattler+2002](http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=894204)), prompting planetary atmospheres to be considered as astrobiological targets ([Smith2013](http://online.liebertpub.com/doi/abs/10.1089/ast.2013.1074)). A denser atmosphere can support a greater number density of suspended particles, raising the possibility of aeroplankton ecologies in the dense supercritical atmospheres of Super-Earths.","created_at":"2015-07-07T08:44:34.680Z","updated_at":"2015-07-09T06:00:38.431Z","zenodo_id":35174,"doi":"http://dx.doi.org/10.5281/zenodo.19667","subject":null,"tags":["astrobiology","exoplanets","microbiology","fluid dynamics","astronomy"],"vote_count":0,"view_count":1,"score":0.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":382,"title":"Beyond Gaia: A Deep-Space Astrometric Mission To Obtain ~1% Parallax Distances at 10 kiloparsecs","sha":"0113b5ca8a28b5333241e3b629e2e910","user_id":null,"state":"published","body":"The [Gaia space telescope](https://arxiv.org/abs/1609.04153) is currently measuring parallax distances for Galactic stars with ~1% precision out to ~1kpc. This will be a revolutionary data set, but order-of-magnitude improvements are possible with a next-generation mission that is already achievable with existing technology.\r\n\r\nEvery parallax distance measurement to-date has used a 2AU baseline, i.e. the Earth-Sun orbital diameter. By launching an astrometric satellite with similar resolution and sensitivity into a deep-space orbit, however, the parallax accuracy could improved in proportion to the orbital major axis. A telescope on an elliptical orbit with a semi-major axis of 20AU would have a parallax accuracy ~10x better than Gaia, allowing 1% distance measurements out to 10kpc, encompassing the Galactic Center and a significant sample of globular clusters. It could also directly measure distances to bright Cepheid variables in the Large Magellanic Cloud, bypassing the first rung in the cosmological distance ladder and its associated systematics.\r\n\r\nLaunching a Gaia-like telescope into a ~20AU orbit would be a significant endeavor, but is achievable with current technology using multiple gravity assists, and is even more compelling with anticipated cost reductions from [reusable heavy-lift launchers](http://www.spacex.com/falcon-heavy). The primary challenge would be the low data-transfer rates from the outer Solar System, although [\u003e10^3 bandwidth increases are planned for the Deep Space Network by ~2030](http://ieeexplore.ieee.org/document/4263976/).","created_at":"2017-05-08T17:24:38.234Z","updated_at":"2017-06-04T16:21:15.949Z","zenodo_id":573378,"doi":"https://doi.org/10.5281/zenodo.573378","subject":null,"tags":["astronomy","astrometry","space mission"],"vote_count":1,"view_count":1,"score":1.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":333,"title":"Searching for \"Tabby's Star\" Analogs in Stripe 82","sha":"534f2373fdf0cd3de184f11a63c4a3ee","user_id":null,"state":"published","body":"[\"Tabby's Star\"](http://adsabs.harvard.edu/abs/2016MNRAS.457.3988B) is an anomalous F star with repeated short-term variability, and a steady [3% dimming over 4-years](http://adsabs.harvard.edu/abs/2016arXiv160801316M). The star spent [~8%](https://twitter.com/Nighthawk_Black/status/762410345411469312) of the Kepler 4-year light curve in the short-duration dimming state. As [LaCourse](/ideas/59bde655f99d29f685702717316e06f3) notes, no analog has been found with [Kepler](http://adsabs.harvard.edu/abs/2010Sci...327..977B). Neither the short- or long-term variability in this object are currently understood so discoveries of additional examples are desperately needed. \r\n\r\n[Stripe82](http://adsabs.harvard.edu/abs/2007AJ....134..973I) was a 10-year multi-epoch photometric catalog by [SDSS](http://adsabs.harvard.edu/abs/2000AJ....120.1579Y), covering ~300 sq deg with 70-90 epochs in *ugriz*. More than 10,000 objects were identified as [variable](http://www.astro.washington.edu/users/ivezic/sdss/catalogs/stripe82.html) with [F star colors](http://adsabs.harvard.edu/abs/2007AJ....134.2398C) from Stripe82, with many likely pulsators or eclipsing binaries. The photometric precision of Stripe82 is sufficient to detect the long-term, non-stationary variability observed from \"Tabby's Star\". Additionally, the 8% duty cycle of short-term dimming events should stand out as several aperiodic outliers in the Stripe82 light curves. Finally, the near-simultaneous multi-band data from SDSS may provide important clues as to nature of this new class of variable star.","created_at":"2016-08-15T21:57:38.909Z","updated_at":"2016-09-01T14:11:31.029Z","zenodo_id":145722,"doi":"http://dx.doi.org/10.5281/zenodo.60308","subject":null,"tags":["kepler","astronomy"],"vote_count":4,"view_count":1,"score":4.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":159,"title":"Sulfur-Bearing Polycyclic Aromatic Hydrocarbons in Interstellar Environments","sha":"c910680b94a3558d7ca47a4fe2046c4a","user_id":555,"state":"published","body":"Polycyclic aromatic hydrocarbons (PAHs) are widely studied in an astrophysical context. Studies have also been performed on nitrogen-bearing PAH species ([Hudgins+2005](http://adsabs.harvard.edu/abs/2005ApJ...632..316H)) and unsuccessful searches have been performed for interstellar nitrogen-bearing PAHs ([Ali-Haïmoud+2015](http://adsabs.harvard.edu/abs/2015MNRAS.447..315A)). Laboratory irradiation of ices containing aromatic hydrocarbons has recently been found to produce oxygen and nitrogen heterocycles ([Materese+2015](http://adsabs.harvard.edu/abs/2015ApJ...800..116M)), though previous astronomical searches for such heterocycles have proven unsuccessful ([Charnley+2005](http://adsabs.harvard.edu/abs/2005AdSpR..36..137C); [Dezafra+1972](http://adsabs.harvard.edu/abs/1972ApL....10....1D); [Kutner+1980](http://adsabs.harvard.edu/abs/1980ApJ...242..541K)). \r\n\r\nHowever, astrochemical studies on interstellar sulfur are quite neglected. Sulfur heterocycles have received little astrochemical attention, despite having been detected inside meteorites ([Krishnamurthy+1992](http://www.sciencedirect.com/science/article/pii/001670379290015B); [Sephton+2002](http://rsta.royalsocietypublishing.org/content/363/1837/2729)). Thiophenes are expected to be stable at interstellar medium (ISM) temperatures ([Guillemin+2010](http://adsabs.harvard.edu/abs/2010cosp...38.3235G)). Observed sulfur depletion does not follow the same pattern as other elements in dust grains ([White+2011](http://adsabs.harvard.edu/abs/2011AAS...21812923W)), and small sulfur compounds like H₂CS are detected in outflows where they are expected to form into more complex species ([Minh+2011](http://adsabs.harvard.edu/abs/2011ApJ...737L..25M)).\r\n\r\nGiven that sulfur and carbon have similar electronegativities and both have extensive chemistries, it is logical to consider heterocyclic sulfur-bearing PAH molecules in astrochemical studies, both experimentally and observationally. Vibrational wavelengths are expected to fall in the 6-9 µm region and at longer wavelengths, where it is well known that many observed emission bands are still poorly understood, with several unidentified or tentatively assigned lines remaining.","created_at":"2015-05-01T10:08:28.636Z","updated_at":"2015-05-14T21:28:20.848Z","zenodo_id":28346,"doi":"http://dx.doi.org/10.5281/zenodo.17292","subject":null,"tags":["astronomy","astrochemistry","molecular astrophysics","sulfur","hydrocarbons","pahs","interstellar medium"],"vote_count":2,"view_count":1,"score":2.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":20,"title":"The Galactic Astigmatism: Constraining the Milky Way Dark Matter Halo Using Ultra-Weak Lensing","sha":"86fd4606f693acf5f94c2808e69804d1","user_id":14,"state":"published","body":"An astigmatism is defined as an asymmetry of the front of the eye, causing light to not focus on a single point. Because the Sun is located ~8kpc off-center in the Milky Way, the integrated mass density of galactic dark matter varies significantly along different lines of sight. As a result, the Milky Way's own DM halo will act as a gravitational lens, magnifying background sources more along lines of sight toward the bulge, and less towards the anti-center. While such lensing may be very weak, it is decidedly non-zero. This ultra-weak lensing may be detected in the number density or apparent brightness of (for example) distant galaxies.\r\n\r\nI note also, this effect has been mentioned only once in the literature that I am aware of, by [Xu and Wu (1987)](http://adsabs.harvard.edu/abs/1987IAUS..124..771X), who speculate the brightness amplification signal may as large as 1e-6 to 1e-5, and possibly detectable in precision CMB data.","created_at":"2015-02-04T22:08:51.143Z","updated_at":"2018-02-19T06:00:28.098Z","zenodo_id":21481,"doi":"http://dx.doi.org/10.5281/zenodo.14841","subject":null,"tags":["astronomy","galaxies","darkmatter","speculation"],"vote_count":13,"view_count":1,"score":13.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":385,"title":"On the Impossibility of Aggregating Individual Cloud Accretion Rates","sha":"83dcae03f859ebcb8e2e710af0ad1b47","user_id":null,"state":"published","body":"The rate of accretion onto the Milky Way (MW) by an individual cloud (or satellite) can be written as dM/dt = M v / R, where M is the cloud mass, v the cloud velocity toward the MW, and R is the distance from the cloud to the MW. Over the time t = R / v a mass M will accrete. This rate can be evaluated for many accreting objects; in the MW about a dozen high-velocity clouds (HVCs) are currently accreting onto the Galactic disk. This said, one cannot sum these accretion rates to determine the MW total rate of growth. For instance, in a scenario with all clouds having mass M, fixed accretion velocity v, and linearly increasing distances R, 2R, 3R, 4R... we would expect a mass M to accrete every R / v, providing a fixed accretion rate of M v / R. Yet by summing the accretion rates of each cloud we find M v / R times the harmonic series 1 + 1/2 + 1/3 + 1/4... This results in an infinite accretion rate. In practice, for currently known HVC distances, the summed accretion rate overestimates by 50% to 100%. ","created_at":"2017-05-24T01:40:31.529Z","updated_at":"2017-06-04T16:21:08.958Z","zenodo_id":582696,"doi":"https://doi.org/10.5281/zenodo.582696","subject":null,"tags":["astronomy","mathematics"],"vote_count":1,"view_count":1,"score":1.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":194,"title":"Shovel-ready for Gaia DR2: Find hypervelocity stars from M31/LMC","sha":"eb7a8d07127ffeba69ff871c1b95d32d","user_id":null,"state":"published","body":"Hypervelocity stars move with extremely large Galactocentric velocities (~1000 km/s) and are found from their anomalous velocities, ages, and metallicities. These stars are thought to have interacted with the supermassive black hole at the center of the Milky Way, given that they are seen moving at large radial velocities relative to the Galactic center. Presumably, the SMBH's in M31 and the LMC (if it exists) should also produce hypervelocity stars, some of which may be moving at large velocities through the Milky Way but not necessarily along the line-of-sight relative to the Galactic center. Step one is to find the stars, next is to measure the mass of the SMBHs in M31 and the LMC by the rate of extragalactic hypervelocity stars.","created_at":"2015-07-24T10:06:02.331Z","updated_at":"2015-07-25T06:00:48.385Z","zenodo_id":39226,"doi":"http://dx.doi.org/10.5281/zenodo.21345","subject":null,"tags":["astronomy","astrophysics"],"vote_count":0,"view_count":1,"score":0.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":null,"attachment_content_type":null,"attachment_file_size":null,"attachment_updated_at":null},{"id":327,"title":"The NASA K2 Mission has yet to observe an analog of Tabby's Star","sha":"59bde655f99d29f685702717316e06f3","user_id":null,"state":"published","body":"Tabby's Star is a main sequence F-class star ([KIC 8462852](https://en.wikipedia.org/wiki/KIC_8462852)) observed by the NASA Kepler spacecraft to undergo significant aperiodic dimming activity across a four year baseline. The light curve was noted as novel amongst a pool of ~150,000 targets through visual inspection performed by participants of the [PlanetHunters.org](https://www.planethunters.org/) project. [Boyajian, et al. (2015)](http://arxiv.org/abs/1509.03622) reported an analysis which attempted to model the enigmatic behavior under various scenarios, with a swarm of exo-comets offered as a potential (but problematic) leading explanation. Subsequent follow-up studies failed to detect a significant IR excess or the presence of close stellar companions. [Wright, et al. (2015)](http://arxiv.org/abs/1510.04606) offered an alternative explanation invoking transits of alien-megastructures, which may not produce a detectable IR excess or radial velocity signal. \r\n\r\nTo date, a robust model to explain the behavior of Tabby's Star remains elusive.\r\n\r\nThe Kepler spacecraft is now observing a series of new ecliptic fields ([K2](http://keplerscience.arc.nasa.gov/)) and has accumulated observations of ~165,000 additional targets. Continued visual inspection of these public data has failed to recover an analog to KIC 8462852. Lack of such a detection suggests that the aperiodic dimming indeed represents a rare astrophysical phenomenon, regardless of the true root cause mechanism involved. Future photometric surveys by K2 and TESS will offer additional opportunity to search for such analogs.\r\n\r\n","created_at":"2016-08-04T03:36:28.126Z","updated_at":"2016-08-15T21:57:38.941Z","zenodo_id":143398,"doi":"http://dx.doi.org/10.5281/zenodo.59494","subject":null,"tags":["astronomy","kepler","astrophysics","crowdsourcing"],"vote_count":2,"view_count":1,"score":3.1,"deleted":false,"muted":false,"tweeted":true,"attachment_file_name":"8462852_LC_img.png","attachment_content_type":"image/png","attachment_file_size":145799,"attachment_updated_at":"2016-08-04T03:36:58.456Z"}]