Extending the Breakthrough Listen nearby star survey to other stellar objects in the field

Brief

Wlodarczyk-Sroka, Garrett, and Siemion (2020) identify 286,998 additional stars residing within the FWHM of GBT L-band (8.4 arcmin), GBT S-band (5.5 arcmin), and Parkes 10-cm (6.4 arcmin) beams already pointed at 1,317 Breakthrough Listen primary targets, using Gaia DR2 geometric distances from Bailer-Jones et al. (2018). The enlarged sample improves the Continuous Waveform Transmitter Rate Figure of Merit (CWTFM) to ~0.055 for GBT, roughly half the prior best of 0.11 from Price et al. (2020), and constrains transmitter prevalence to <0.066% within 50 pc (EIRP ≥ 10^13 W) and <0.039% within 200 pc (EIRP ≥ 2.5×10^14 W). The authors caution that narrow-band results beyond ~200 pc at L-band frequencies are degraded by interstellar scintillation, and they introduce spectral-class-stratified prevalence limits for the first time, including a recommendation that white dwarf systems receive greater SETI attention.

Metadata

Category

Search

Venue

Monthly Notices of the Royal Astronomical Society

Type

Peer-reviewed

Year

2020

Authors

B. S. Wlodarczyk-Sroka, M. A. Garrett, A. P. V. Siemion

DOI

10.1093/mnras/staa2672

arXiv

2006.09756

Access

Open access

Length

15.1 M

Programs

Breakthrough Listen

Instruments

Green Bank Telescope GBT L-band, Green Bank Telescope GBT S-band, Parkes 10-cm receiver

Data sources

Gaia DR2, Bailer-Jones et al. (2018) geometric distance catalogue, Breakthrough Listen Price et al. (2020) target list

Tags

SETI, technosignature, radio-SETI, stellar-survey, transmitter-prevalence, narrowband

Key points

• Sample expanded 219× from 1,327 to 288,315 stellar objects by cross-matching all Breakthrough Listen GBT and Parkes pointings with Gaia DR2 geometric distances; distances span 1.3 pc to ~9,987 pc.p.4
• Fewer than 0.0660+0.0004−0.0003% of stellar systems within 50 pc host continuous transmitters with EIRP ≥ 10^13 W, the tightest limit published at the time, improving on Enriquez et al. (2017)'s <0.1%.p.1
• At 100 pc, <0.061+0.001−0.003% of systems for EIRP ≥ 6.5×10^13 W; at 200 pc, <0.039+0.004−0.008% for EIRP ≥ 2.5×10^14 W.p.7
• GBT CWTFM reaches ~0.0551 at EIRP_min = 10^12 W, compared to 0.11 from Price et al. (2020); the improvement comes entirely from beam-filling background and foreground stars, not new observations.p.6
• HIP1692, included in the BL sample as a 23.0 pc target, is placed by Gaia DR2 at ~769 pc, a 33× distance error originating in the 1997 Hipparcos main catalogue.p.5
• 75 Price et al. (2020) primary targets have no Gaia DR2 counterpart, typically because they are bright nearby stars with proper motions exceeding 0.6 arcsec/yr, a known Gaia incompleteness regime.p.5
• Interstellar scintillation at ~1 GHz becomes problematic beyond ~200 pc, broadening narrow-band signals and causing amplitude intermittency scaling as ν^−2; S-band data are less affected.p.7
• White dwarf systems are proposed as underexplored SETI targets: cooling-phase habitable zones can persist stably over billion-year timescales, and surviving debris disks and planets have been detected around them.p.8

Verbatim

• “Enlarging the sample from 1327 to 288315 stellar objects permits us to achieve substantially better Continuous Waveform Transmitter Rate Figures of Merit (CWTFM) than any previous analysis, and allows us to place the tightest limits yet on the prevalence of nearby high-duty-cycle extraterrestrial transmitters.”

  p.1

• “Until now, the impact of the additional foreground and background stars has largely been ignored, partly because the distances to these stars were typically unknown.”

  p.2

• “While it seems counter-intuitive that a stellar system at the end of its life cycle might provide conditions that are hospitable to life, in the cooling phase these stars can maintain relatively benign environments that are expected to be stable on time scales of many billions of years.”

  p.8

• “We very much prefer this open-ended approach to SETI surveys, as opposed to highly focused approaches centred on familiar solar-type stars.”

  p.8

Most interesting

• The 10^13 W EIRP detection threshold approximates Earth's current total energy consumption, a transmitter at this power must broadcast continuously and directionally toward us to be detectable at 50 pc with GBT.
• A single GBT L-band pointing (FWHM ~8.4 arcmin) can cover dozens of Gaia-catalogued stars simultaneously; the field around primary target HIP109427 alone contains 45 additional Gaia sources within the beam.
• The historical Project Phoenix survey scored a CWTFM of ~49; this paper's best GBT result reaches ~0.045, roughly a 1,000× improvement in figure of merit using the same underlying observations, just analyzed differently.
• HIP1692 was placed in the Breakthrough Listen nearby-star sample on the assumption it was 23 pc away; Gaia DR2 reveals it is ~769 pc distant, meaning it radically overstates EIRP sensitivity for that pointing.
• The Parkes telescope's CWTFM remains far worse than GBT (1.38 vs. 0.055 at EIRP_min = 10^12 W) even after beam-filling augmentation, reflecting its smaller primary beam and fewer background stars captured per pointing.
• The paper notes that extended objects, likely foreground galaxies, also appear within the GBT primary beam for some pointings, raising the possibility of extragalactic SETI limits from the same dataset, though this is not pursued.