midcycle submision 1-1363034400Assigned ID: 1154
----- Attempting Submission 1 (Fri Sep 27 21:36:22 GMT 2019) -----
HST Phase I Proposal 1154 (VHS_midcycle_submitted.pdf.apt) successfully submitted.
Receipt: # 1154-1Phase II re-submit
Target coordinates updated
PCS-mode corrected for Visit 02-07-830779061----- Attempting Submission 1 (Sat Nov 16 00:53:53 GMT 2019) -----
----- Attempting Submission 2 (Thu Apr 30 16:19:45 GMT 2020) -----Mapping Clouds on a Variable Planetary-Mass CompanionTime-series observations of rotational modulations in brown dwarfs and planetary-mass companions are a powerful tool to characterize ultra-cool atmospheres. In particular, the chromatic variations in variability amplitudes place strong constraints on the properties of heterogeneous clouds, and the temporal evolution of the lightcurve directly probes the physical processes that drive atmospheric circulation. We recently discovered that a planetary-mass companion has a J-band variability of more than 22%. This is so far the largest variability amplitude discovered for a PMC and offers the best opportunity for a detailed characterization of an ultra-cool, very low surface gravity atmosphere.
We propose fifteen orbits HST/WFC3/IR G141 time-series observations of this companion. We have developed a seven-visit strategy to establish a time baseline of ~60 hrs, which will continuously monitor our target for two rotation periods. These spectrally-resolved rotational modulated variability measurements will provide a measure to distinguish and characterize multiple aspects of atmospheric heterogeneity. This precise light curve will determine this object's circulation patterns. This object will also be monitored by Spitzer at 4.5-micron band in the near future through a compleetary program. This proposal and the Spitzer observations together will determine this object's mid- to long-term atmospheric evolution. Importantly, this object is also a JWST ERS program target which will result in a high-quality 2 to 28 micron JWST spectrum. The atmospheric structures and their temporal evolution revealed in these observations will provide crucial context for interpreting its JWST spectrum.Instrument
Measuring VHS1256b's rotational modulations in the 1.4-micron water absorption band is an essential component of our experiment.
Therefore, we choose WFC3 and its G141 grism because of their wavelength coverage and excellent photometry performance. As shown in the Cycle25 observations of VHS1256b, WFC3/G141 spectral time-series detected VHS1256b's NIR color and water depth modulations at high fidelity. We will adopt RECTE model \citep{Zhou2017} to correct the systematics in the time series data and ensure full data usage. We opt to use the 256x256 subarray to avoid in-orbit buffer downloads. For wavelength calibration, at the beginning of each orbit, we will include two or three short direct-imaging observations in the F132N filter with the same aperture and subarray as the spectroscopic exposures.
Visit planning
We split the 15 orbits observation into seven visits. Because the instrument systematics is more significant at the beginning of the observations, we plan three orbits for the first visit and two orbits for the remaining six visits. We allow two- to four-orbit gaps between any adjacent visits in the first four visits and five to seven orbit gaps between adjacent visits in the remaining three visits. There are three main purposes for this design. First, it ensures a fine temporal sampling of the first rotation period, which enables detailed analysis and mapping of the heterogeneous atmosphere of VHS1256b for one full rotation. Second, this design provides a total time baseline of ~60 hr, which is sufficient to cover two rotation periods of VHS1256b and determine the atmospheric evolution on this timescale. Third, scheduling flexibility is increased under this design compared to a continuous 15 orbits long sequence. The impact of passing the South Atlantic Anomaly can thus be mitigated.
Exposure time and SNR estimates
We estimate exposure time and the expected signal-to-noise ratio based on the archival spectrum of VHS1256b. For spectroscopic exposure, we will use SPARS25 sample sequence with NSAMP of 11, which is equivalent to an exposure time of 223 seconds. Based on the WFC3 exposure time calculator estimates, this exposure time will yield an SNR of 70 per spectral element or SNR>500 for the 1.1 to 1.7 micron broadband at a cadence of 240 seconds. The SNR and cadence are comparable to previous HST/WFC3/G141 light curves of variable brown dwarfs that are used for top-of-atmosphere map retrieval.
Roll angle constraints
To reduce the effect from flat field uncertainties, we request the telescope roll angle to be fixed for the entire observation and also request no dithering to be applied. VHS1256b and its host binary are separated by 8.1 arcsec with a position angle of 218 degrees. The most ideal telescope position angle (U3) is 173.1 or 353.1 degrees. To avoid host binary spectral trace contaminating that of the companion, we request at least 30 pixels separation between the traces of the companion and primary spectra, and thus request the telescope orientation (U3) to be 113 to 233 degrees or -67 to 53 degrees. If scheduling permits, the telescope roll should be as close to 173.1 or 353.1 degrees as possible. Also if possible, telescope pointing for the entire seven visits observations should be kept the same.PI Yifan Zhou is an expert on HST/WFC3 time-series observations of brown dwarfs and planetary-mass companions. Zhou has led multiple studies of rotational modulation measurements of planetary-mass companions.
Co-I Bowler is an expert on observations of directly-imaged exoplanets and their characterizations.
Co-I Apai is an expert on atmospheric characterizations of brown dwarfs and exoplants. Apai is the leading expert of using space-based oobservations for rotational modulaiton of ultra-cool substellar objects.
Co-I Morley is an expert on cloud formation models of ultra-cool atmospheres.
Co-I Bryan is an expert on measuring the rotation of ultra-cool objects
Co-I Benneke is an expert on HST/WFC3 time series observations and characterizations of exoplanet atmospheres.
Co-I Kataria is an expert on atmospheric circulation models of brown dwarfs and exoplanets.
co-I Skemer is an expert on direct-imaging observations and atmospheric characterization of giant exoplanets. Skemer is the co-PI of the JWST ERS program that includes our target.Planets and Planet FormationStellar Physics6falsefalsefalsefalsefalsefalsefalseSUPPORTEDSTARBROWN DWARFEXTRA-SOLAR PLANETJ2000J2000J=16.66 magWFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141WFC3/IRSpectroscopicNoNoG141/private/var/folders/rt/jky8v5311rg3q7171ssfr0f00000gn/T/OPVisit7932761627485080219.xmlTiming link suitability includes constraints ([02 After 01 by 0.1 to 0.3 days])Hst Same orient constraint links visits (01 02 03 04 05 06 07)The scheduling data was generated by CASM 28.0.CASM was run in Phase II mode.(Tic-Visit
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