TESS reveals that the nearby Pisces-Eridanus stellar stream is only 120 Myr old

Pisces-Eridanus (Psc-Eri), a nearby ($d$ $\simeq$ 80-226 pc) stellar stream stretching across $\approx$120 degrees of the sky, was recently discovered with Gaia data. The stream was claimed to be $\approx$1 Gyr old, which would make it an exceptional discovery for stellar astrophysics, as star clusters of that age are rare and tend to be distant, limiting their utility as benchmark samples. We test this old age for Psc-Eri in two ways. First, we compare the rotation periods for 101 low-mass members (measured using time series photometry from the Transiting Exoplanet Survey Satellite, TESS) to those of well-studied open clusters. Second, we identify 34 new high-mass candidate members, including the notable stars $\lambda$ Tauri (an Algol-type eclipsing binary) and HD 1160 (host to a directly imaged object near the hydrogen-burning limit). We conduct an isochronal analysis of the color--magnitude data for these highest-mass members, again comparing our results to those for open clusters. Both analyses show that the stream has an age consistent with that of the Pleiades, i.e., $\approx$120 Myr. This makes the Psc-Eri stream an exciting source of young benchmarkable stars and, potentially, exoplanets located in a more diffuse environment that is distinct from that of the Pleiades and of other dense star clusters.


INTRODUCTION
Star clusters at least 1 Gyr in age are rare, and tend to be located at large distances from Earth (e.g., Dias et al. 2002;Kharchenko et al. 2005). This is a shame, because such clusters serve as critical benchmarks for stellar astrophysics. Recently,  announced the discovery of a stellar stream that stretches 120 • across the sky, and spans ≈400 pc in space. This discovery was made possible by the precise astrometry, radial velocities (RVs), and photometry included in the Gaia mission's second data release (DR2; Gaia Collaboration et al. 2018a). Discovery of the Pisces-1 The stream was undesignated in . The authors of the discovery paper suggested the name "MAF-1" for the stream (S. Meingast, priv. comm.); however, this is very different from the nomenclature for nearby associations (e.g. de Zeeuw et al. 1999;Torres et al. 2008). This acronym could be confused with two acronyms already in the Dictionary of Nomenclature of Celestial Objects (http://cds.u-strasbg.fr/cgi-bin/Dic-Simbad; Lortet et al. 1994)-[MAF2004] and [MAF2009]-the latter of which is used for members of the open cluster NGC 7062 (Molenda-Żakowicz et al. 2009), or as an abbreviation of the Maffei galaxies or Maffei Group of galaxies (e.g. Fingerhut et al. 2007). Two of the main concentrations of the stream's members are in the constellations Eridanus (clump 1) and Pisces (clump 3), and the group's convergent point (α, δ ≃ 42. • 6, −20. • 0; ICRS) lies in Eridanus as well. As we find in our analysis that the group is more analogous to an older version of an OB association, similar to other expansive nearby stellar associations like Sco-Cen and Tuc-Hor, we combine the two prominent constellation names and refer to it as the "Pisces-Eridanus stream" or Psc-Eri. Note-These distance moduli only account for distance, and do not include visual extinction. a The Pleiades age has been constrained with lithium depletion boundary to 125-130 Myr by Stauffer et al. (1998) and 115 ± 5 Myr by Dahm (2015). Recent isochrone analyses by Gossage et al. (2018) found 110-160 Myr; Cummings & Kalirai (2018) found 115-135 Myr. We adopt 120 Myr for this work.
given its combination of old age (≈1 Gyr) and proximity (d = 129±32 pc from Earth; median and standard deviation of the 256 published members; the full range is d ≃ 80-226 pc). For context, we list the distance moduli for notable benchmark open clusters along with their ages in Table 1. Figure 1 plots the age and distance to a selection of clusters with measured rotation periods (P rot ), which further highlights how remarkable and useful a 1 Gyr cluster this close to Earth would be.
If Psc-Eri's age is truly 1 Gyr, it would be the oldest coeval stellar population within 300 pc. This would open up many avenues for research that are difficult or impossible to pursue with the 1 Gyr-old benchmark cluster NGC 6811 (Sandquist et al. 2016;Curtis et al. 2019), currently the only open cluster of this age we have been able to study in detail. For example, Meibom et al. (2013) discovered two sub-Neptune exoplanets in NGC 6811, but these are too faint for efficient RV follow-up. It is also challenging to measure chromospheric Ca II H & K activity indices for FGK stars in NGC 6811: those stars are faint (a solar twin is V ≈ 15), and the interstellar Ca II H & K contamination is difficult to mitigate (Curtis 2017). Finally, Psc-Eri could be an interesting test case for demonstrating the chemical tagging technique needed for Galactic archaeology (Freeman & Bland-Hawthorn 2002).
Given the potential value of this population of stars, it is important to examine its age to see if it can serve as a benchmark for old stars. A similar exercise with the purportedly old nearby cluster Ruprecht 147 proved very fruitful (Curtis et al. 2013;Curtis 2016)   This work Figure 1. Age versus distance for a selection of benchmark star clusters with rotation period measurements. The distance to the Psc-Eri stream is shown as a red point marking the median and a red line showing the range. If this stream is really ≈1 Gyr in age, it would become a critical target for rotation/activity studies and an important benchmark for stellar astrophysics. By comparing rotation periods in Psc-Eri to those in the clusters shown as colored stars, and by re-examining its color-magnitude diagram, we demonstrate that it is closer to ∼100 Myr old. exploration of another candidate old cluster, Lodén 1, showed that it did not exist (Han et al. 2016).
We use gyrochronology, the age-dating method based on stellar rotation and magnetic braking (Barnes 2003;Soderblom 2010), to test the existence and coevality of the Psc-Eri stream. Coeval stars form well-defined sequences in their color-period diagrams, analogous to the main sequence in a color-magnitude diagram (CMD). But color-period sequences are much more sensitive to age, as the full sequence evolves measurably in time as stars spin down, while only the massive end of the main sequence shows significant evolution in temperature and luminosity. If the stars are coeval, a gyrochronology analysis will also yield a precise age for the Psc-Eri stream. We conduct this experiment in Section 2, where we extract and analyze light curves for 101 members of the stream observed by TESS. We find that the resulting P rot distribution precisely overlaps the Pleiades distribution, making it ≈120 Myr old.
In Section 3, we reinterpret the stream's CMD by noting that Gaia DR2 measured RVs for stars with T eff 7000 K, which biased the Meingast et al. (2019) membership census. The Psc-Eri stream's CMD closely matches that of the Pleiades, except that its membership is truncated due to this RV bias. Combining Gaia DR2 data with literature RVs, we identify 22 new can-didates that are warmer than the stars in the  sample, and another 12 that lack RVs but are co-moving in proper motion within 10 pc of known members. These stars closely follow the upper main sequence of the Pleiades, providing further evidence of the Psc-Eri stream's young age. We also briefly discuss the stream's formation in Section 3, before concluding in Section 4.   published a list of 256 candidates members of the Psc-Eri stream. We used the Web TESS Viewing Tool (WTV) 2 to identify stars observed during Sectors 1-5, and we found 154 with data from at least one sector. We downloaded 20×20 pixel cutouts of the FFI images centered on each target using the TESSut tool hosted at MAST (Brasseur et al. 2019). 3 Next, we used the IDL procedure aper.pro from the IDL Astronomy User's Library (Landsman 1993) to perform aperture photometry on all epochs in the image stack produced by TESScut. We used a circular aperture with a 3 pixel radius (≈1 ′ based on TESS 's ≈21 ′′ pixel scale).
The resulting light curves overwhelmingly showed clear spot modulation with relatively large amplitudes and short periods compared to our expectations from the 1 Gyr NGC 6811 data from Kepler Meibom et al. 2011b). We were able to measure P rot without performing any additional calibration on these light curves. Figures 2 and 3 show examples of TESS light curves for stream members produced following this simple procedure.

The Color-Period Diagram
We measured rotation periods for 101 stars using Lomb-Scargle periodograms (Scargle 1982;Press & Rybicki 1989). After extracting each light curve and computing the periodogram, we visually inspected the results (see Figures 2 and 3) to ensure the accuracy of our measurements. On only three occasions did we double the Lomb-Scargle period to correct for a 1/2-period harmonic error, which we visually identified by noticing asymmetry in the depths of alternating minima and other subtle morphological asymmetries.
Eleven stars were observed twice, in neighboring sectors, and for these we find consistent periods across sectors. Figure 3 shows an example where we stitched the light curves from two sectors together, and found a more precise period than attained from either sector separately (based on the width of the periodogram peak). Stitching the light curves together was simplified by the fact that multiple maxima and minima were captured in each sector, which meant that no reference stars were needed to normalize the light curves from each sector.
The bottom left panels of Figures 2 and 3 plot Gaia DR2 color (G BP − G RP ) versus P rot for our sample. The majority of the stars follow a common sequence, indicating that they are coeval.

A Gyrochronological Age
Gyrochronology only requires as input the mass of a star (or a proxy like temperature or color) and its P rot . There are a variety of empirical gyrochronology models available, including those of Barnes (2003), Barnes (2007) and its various re-calibrations (e.g., Mamajek & Hillenbrand 2008;Angus et al. 2015), and Barnes (2010). There are also theoretical models that pair stellar evolution with a magnetic torque law to predict angular momentum evolution (e.g., van Saders & Pinsonneault 2013;Matt et al. 2015;Gallet & Bouvier 2015). However, no model has been published that can explain all of the cluster rotation data (see Curtis et al. 2019;Douglas et al. 2019;Agüeros et al. 2018). Instead, we suggest that the best way to constrain the age of the Psc-Eri stream with gyrochronology is by comparing its P rot distribution directly to the distributions measured for benchmark clusters.

The Benchmark Cluster Sample
The Pleiades is ≈120 Myr old (Stauffer et al. 1998, see also Table 1), has a metallicity of [Fe/H] = +0.03 dex (Soderblom et al. 2009) and an interstellar reddening of E(B − V ) ≈ 0.044 (A V = 0.14; Taylor 2006). P rot for 759 members were measured by Rebull et al. (2016a) from K2 light curves collected during its Campaign 4 (see also Rebull et al. 2016b;Stauffer et al. 2016). We cross-matched this list with Gaia DR2 and filtered out stars that were more than 0.375 mag discrepant from the single-star sequence, which we defined with the Gaia Top-Example TESS light curve for Gaia DR2 5029398079322118912, which was observed during Sector 3. The length of the red line at the top left is the duration of one cycle (i.e., P rot ). Middle left-The Lomb-Scargle periodogram shows P rot = 6.64 d. In some cases, the periodogram did not produce an accurate measurement, so we calculated P rot by fitting the timing of successive maxima and/or minima, illustrated by the red line in the top panel. Middle right-This phase-folded light curve visually validates the periodogram analysis. Bottom left-The color and period for this star (red star) are plotted along with the full rotator sample for the Psc-Eri stream (black points). The Gaia DR2 T eff is also provided (Andrae et al. 2018). Bottom right-The 20×20 pixel cutout of the TESS full frame image for this target, encircled with a three pixel radius aperture used to extract the light curve (red circle). Versions of this figure for every target analyzed are available as an electronic figure set in the online Journal (101 images) .  Figure 2, but for the 11 stars with two sectors of data. Top-The TESS light curve for Gaia DR2 4984094970441940864, which was observed during Sectors 2 (blue) and 3 (red). The length of the black line at the top left is the duration of one cycle (i.e., P rot ). Middle left-Lomb-Scargle periodograms for Sector 2 (blue), Sector 3 (red), and the joint light curve (black). While we find the same period from each individual sector, the periodogram peak is noticeably narrower for the joint light curve. Middle right-The phase-folded light curves for each sector show that the light curves can be reliably merged by simply stitching them together with no additional calibration needed (for these rapid, active stars, at least). Bottom left-The color and period for this star (red star) are plotted along with the full rotator sample for the Psc-Eri stream (black points). Bottom right-The 20×20 pixel cutout of the TESS full frame image for this target, encircled with a three pixel aperture used to extract the light curve (red circle).  Right-Similar to the previous panel, and now including rotation periods for 101 members of the Psc-Eri stream (red stars) identified by , and measured by us from TESS FFI data. Approximate spectral types are listed at the top of each panel for reference. Clearly, the rotation period distribution for this stream favors an age much younger than 1 Gyr. We infer an age of ≈120 Myr for the Psc-Eri stream based on its similarity with the Pleiades.
et al. 1999). We also removed stars with absolute differences in proper motion relative to the cluster median greater than 3 mas yr −1 , corresponding to ≈2 km s −1 at 136 pc, or four times the internal velocity dispersion (Madsen et al. 2002). Praesepe is 670 Myr old  and has a metallicity of [Fe/H] = +0.15 dex (Cummings et al. 2017). P rot for 743 members were amassed from the literature and measured from K2 Campaign 5 light curves by Douglas et al. (2017). Douglas et al. (2019) crossmatched this list with DR2 and filtered out stars that failed membership, multiplicity, and data quality criteria, leaving us with 359 single star members.
The 1 Gyr-old NGC 6811 cluster has a solar metallicity (Sandquist et al. 2016). P rot for 171 likely single-star members were recently measured by Curtis et al. (2019), more than doubling the size of the rotator sample from Meibom et al. (2011b), and extending its lower mass limit from ≈0.8 M ⊙ to ≈0.6 M ⊙ .

Stellar Properties
Gaia DR2 provided effective temperatures (T eff ) for ≈1.61×10 8 stars with 3000 T eff 10, 000 K and G < 17 mag (Gaia Collaboration et al. 2018b) via the Apsis pipeline (Bailer-Jones et al. 2013). The DR2 photometry is very precise, but the Apsis temperatures are severely affected by interstellar reddening. However, this bias can be mitigated by de-reddening the photometry for each cluster sample prior to converting it to T eff . We employ an empirical color-temperature relation to convert the de-reddened Gaia DR2 (G BP −G RP ) 0 color to T eff . Our relation is a polynomial fit to benchmark stellar data assembled from the catalog of spectroscopic properties for the solar-type stars (4700 < T eff < 6700 K) targeted by the California Planet Survey (Brewer et al. 2016

The Psc-Eri Stream is Coeval with the Pleiades
In the left panel of Figure 4, we present the P rot distribution for likely single-star members of our three benchmark open clusters as a function of T eff . In the right panel, we add the the P rot distribution for the Psc-Eri stream. The Psc-Eri stream's P rot distribution is nearly indistinguishable from that of the Pleiades. In particular, the slow, converged sequences for each system are remarkably consistent.
There are a few differences. The Psc-Eri stream has more outliers at periods intermediate to the slow sequence and the rapid ≈1 d rotators. This could be due to poor binary rejection, or slight differences in ageif younger than the Pleiades, those stars could still be converging. In addition, the Pleiades sample extends to much cooler T eff . As we discuss in Section 3.1, this is because RVs were used to identify members of the Psc-Eri stream, and DR2 does not provide RVs for such cool and faint stars. Finally, the warmest stars in the Psc-Eri stream (T eff 6100 K) appear to be rotating subtly and systematically faster than their analogs in the Pleiades. Perhap this also indicates that the stream is slightly younger than the Pleiades.
In contrast, the late-F to early-K dwarfs are, again, remarkably consistent. The slow, converged sequences for both populations are well-described by a line of constant Rossby number. 4 Focusing on the stars with 4600 < T eff < 6100 K that have converged to within 25% of the slow sequence, the median and standard deviation of the Rossby number for the 43 Pleiades in this sample is Ro = 0.29 ± 0.03, and we find Ro = 0.29 ± 0.02 for the 39 stream members meeting the same criteria. These values are incredibly precise, and strikingly similar. The unavoidable conclusion is that the Psc-Eri stream is ≈120 Myr in age. 5

REVISITING THE PSC-ERI STREAM'S CMD
The left panel of Figure 5 is the CMD for the stream, 6 together with members of the Pleiades (Gaia Collaboration et al. 2018b) and NGC 6811 Curtis et al. (2019). We also include PARSEC isochrones (Bressan et al. 2012) appropriate for the Pleiades (130 Myr, solar metallicity), and NGC 6811 (1 Gyr, solar metallicity).

The Apparent Absence of a Main-Sequence
Turnoff Is a Problem The absence of Psc-Eri members warmer than T eff ≈ 7760 K on the main sequence would seem to favor an older age for the stream. However, as  pointed out, the stream lacks a clear main-sequence turnoff (MSTO). This is a problem: if the Psc-Eri stream is 1 Gyr old, there should be a welldefined MSTO ( Figure 5 shows the case of NGC 6811). If the stream is young, the higher-mass stars should fol-4 Ro = P rot /τ . We used the formula for convective turnover time, τ , from Cranmer & Saar (2011). 5 We performed similar comparisons with M35 (NGC 2168, 150 Myr; Meibom et al. 2009) and M34 (NGC 1039, 220 Myr; Meibom et al. 2011a), and found that the Psc-Eri P rot distribution was most consistent with that of the Pleiades. Specifically, the slow sequences for the older M35 and M34 clusters are converged to lower masses and longer periods (see figure 12 in Stauffer et al. 2016), whereas the slow sequences for Psc-Eri and the Pleiades share a common maximum P rot of ≈8.5 d, where the distributions turn over toward more rapid rotation toward lower masses and cooler temperatures. 6 We adopt d = 1000/̟ to estimate distances for each star, and so calculate absolute magnitudes as M G = G − 5 log 10 (100/̟), with units of pc and mas for d and ̟. low the Pleiades main sequence. Either way, these stars should exist somewhere in the CMD, but they are either missing from the stream or missing from its membership catalog.
Identifying members of most star clusters is facilitated by their spatial overdensity and distance from Earth: proper motion are sufficient, and RVs are not strictly needed for finding candidate members. Identifying members of moving groups, stellar streams, and very nearby clusters (e.g., the Hyades) is more difficult because 3D kinematics are required. Accordingly,  used RVs to identify candidate Psc-Eri stream members. But the Gaia Radial Velocity Spectrometer (Soubiran et al. 2013;Cropper et al. 2018) provided measurements for stars with 3550 T eff 6900 K  in DR2. This data limitation means that the  criteria automatically precluded the identification of the MSTO for the Psc-Eri stream.
The left panel of Figure 5 plots the Pleiades membership (Gaia Collaboration et al. 2018b), and highlights those with DR2 RVs. The CMD for this Pleiades RV sample looks identical to the  membership for the Psc-Eri stream. We conclude that selecting members while requiring Gaia RVs will exclude warmer members, if they exist (as well as the coolest, lowest-mass stars and hot white dwarfs).

New, Massive Candidate Psc-Eri Members Support a Young Age
If the Psc-Eri stream is the same age as the Pleiades, we should be able to identify hotter, more massive stars that are spatially and kinematically consistent with the  members. To this end, we queried DR2 for stars with G < 10 mag, (G BP − G RP ) < 0.5, and M G < 3 mag, which returned 435,601 stars. We trimmed this to 6851 stars by selecting those consistent with the stream in R.A. versus µ α cos δ, R.A. versus π, and decl. versus µ δ diagrams. Next, we searched SIMBAD (Wenger et al. 2000) for RV measurements for these stars. We found 2332 matches for which we could then calculate 3D galactic U V W velocities.
Of  justified by the fact that hotter, rapidly rotating stars will have less precise RVs than those for the FGK dwarfs reported in DR2. 8 We also searched the 10 pc volume around every  member for co-moving neighbors, according to the proper motion criterion ∆µ < 2 mas yr −1 , and found 377 co-moving candidates, including ten high-mass stars. Oh et al. (2017) performed a similar exercise to identify co-moving pairs and larger groups using a more sophisticated algorithm applied to the Tycho-Gaia Astrometric Solution (TGAS; Michalik et al. 2015) catalog, released with Gaia DR1 (see also Andrews et al. 2017). While the Psc-Eri stream was not identified as a co-moving system in their analysis, they did identify seven high-mass stars as co-moving part- ners with members from , adding two unique stars to our high-mass candidate list (12 comoving neighbors in total). Table 4 lists our 34 highmass candidate members.
The right panel of Figure 5 shows the CMD for the Pleiades members together with the Meingast et al. list has 43 more that are brighter and warmer than T eff ≈ 7000 K RV cutoff, and we found 34 candidates in the stream.
Two of the five brightest candidates in the CMD are expected to have atypical photometry and should be excluded from an isochronal age analysis (Cummings & Kalirai 2018): according to SIMBAD, λ Tau is an Algoltype eclipsing binary and omi Aqr is a Be star. Focusing on the blue edge of the upper main sequence, the Psc-Eri stream is appears approximately coeval with the Pleiades. The 80 Myr and 130 Myr PARSEC isochrones shown in in the right panel of Figure 5 do not diverge appreciably in color at the luminosities covered by the Pleiades and Psc-Eri stream samples. We postpone a precise isochronal analysis until we can validate the membership of our high-mass candidates with new RV measurements.

How Did the Psc-Eri Stream Form?
Meingast et al. (2019) estimated the Psc-Eri stream progenitor cluster mass to be ≈2000 M ⊙ , noted that the Hyades initial mass has been estimated to be ≈1700 M ⊙ , and concluded that since the Hyades still has a gravitationally bound core, the Psc-Eri stream, which has been dispersed, must be older.
Indeed, if it were truly 1 Gyr old, the Psc-Eri stream would have had to be born as a dense cluster analogous to the Pleiades, Hyades, or NGC 6811, to survive for so long before disrupting. However, given that we now know that it is actually ≈120 Myr, this constraint on the stream's birth conditions is unnecessary. In their figure A.1 and table A.2,  identified four main clumps within the stream. These clumps are presently separated by ≈160 pc, and this clumpiness is similar to that seen in the much younger Tuc-Hor (Kraus et al. 2014) or Sco-Cen associations (Preibisch & Mamajek 2008;Pecaut & Mamajek 2016;Wright & Mamajek 2018), which are gravitationally unbound.
We suggest that the members of the Psc-Eri stream were not formed in a dense cluster but instead formed in a more decentralized fashion, similar to these OB associations. If correct, this would resolve two challenges to our young age result: 1. Why does the stream not have a well defined core?
Our answer is that it never had one, but instead formed several smaller clumps.
2. How could a 120-My-old cluster disperse its stars across 400 pc with such a low internal velocity dispersion? The ends of stream had a head start, as they were born separated in space, and the members of each subgroup dispersed from there.
According to the Gaia Collaboration et al. (2018b) membership lists, the Pleiades has 611 members within 5 pc of its center, and the Hyades has 195 members in the same size volume. In contrast, we suggest that the stream formed multiple approximately coeval clumps; therefore, each zero-age core density is much less than expected based on the present-day star count.
If we are correct, this would mean that the stream provides an environment to its stars that is distinct from that of the Pleiades, and which might be representative of a more common star formation channel in the Galaxy than dense cluster formation (e.g., Clark et al. 2005). That would make the Psc-Eri stream an excellent target for exoplanet searches, which have so far turned up nothing for the Pleiades (Gaidos et al. 2017).  discovered an exciting new stellar stream located relatively nearby (d ≃ 80-226 pc). We were intrigued by its apparently old age (≈1 Gyr), as this would make it a critical target for the calibration and validation of a variety of age-dating techniques, including stellar activity, rotation, lithium depletion, and other chemical clock techniques.

CONCLUSION
Using new time series photometry from TESS, we measured P rot for 101 of the Psc-Eri stream's members. We found that the majority of these members actually overlap with the P rot distribution for the Pleaides, indicating that the Psc-Eri stream is only ≈120 Myr old.
By contrast to the CMD for the ≈1 Gyr old cluster NGC 6811, the  CMD for the Psc-Eri stream lacked a MSTO. We concluded that this is because the Psc-Eri stream is young, and that the more massive stars that would otherwise occupy the MSTO are warmer than the T eff 7000 K cutoff for the Gaia DR2 RV dataset; i.e., warmer stars could not be detected in DR2 as members by  because they lack 3D kinematics. We expanded the search for these missing members by pairing DR2 with RV measurements in the literature tabulated by SIMBAD, and also by searching for co-moving neighbors to the known members. We found 34 candidates that closely track the upper main sequence of the Pleiades, further strengthening our finding of a young age for the Psc-Eri stream.
There is one point on the Psc-Eri stream's CMD consistent with an old age: the evolved 42 Cet triple system. Given the indisputably young age for the Psc-Eri stream we found with gyrochronology, we suspect it is an interloper.  estimated that the stream was formed with a total stellar mass similar to the Hyades. The Hyades has retained a dense cluster structure (with tidal tails; Meingast & Alves 2019), as has the Pleiades, while the stream is diffuse, with an elongated structure spanning 400 pc with four clumps. We argued that rather than being evidence for an older age, this structure indicates that the Psc-Eri stream's stars did not form in a dense cluster environment, but instead in the more decentralized fashion typical of OB associations.
If true, the Psc-Eri stream could become a valuable benchmark system for comparing environmental impact relative to the Pleiades, and for examining how photoevaporation sculpts planet sizes. To date, no planets have been found in the Pleiades (Gaidos et al. 2017). The stream thus presents a new opportunity to search for Pleiades-aged planets. Indeed, a Psc-Eri member has already been identified as a planet candidate host with TESS. 10 This is the first gyrochronology study using TESS data, and it confirms that TESS will be an exciting mission for stellar astrophysics. This is especially true given how TESS records and releases FFI data. The existence of this stream was not known prior to the TESS Cycle 1 call for proposals, and yet the FFI data were ready for us to analyze immediately following the announcement of the stream's discovery by . This is also the first time a stellar stream has been age-dated using gyrochronology, and our work demonstrates the potential for gyrochronology to serve as a powerful tool for Galactic archaeology.  10 First noted by Elisabeth Newton as a Psc-Eri member (priv. comm.), TOI 451 is a G dwarf with T eff ≈ 5530 K (Gaia DR2 4844691297067063424, CD−38 1467, TIC 257605131). Our analysis of the TESS 2 min light curves from Sectors 4 and 5 reveals two sets of transits, suggesting that TOI 451 hosts two planets with P orb,b ≈ 9.19 d and P orb,c ≈ 16.36 d. Follow-up efforts to rule out false positive scenarios and validate the planetary system are being coordinated by the TESS Hunt for Young Moving group Exoplanets collaboration (THYME).    GRP); R.A., decl., (GBP − GRP) G, and MG = G − 5 log 10 (100/π) are from Gaia DR2; P rot is measured from TESS FFI data (days). The notes indicate if a star is converged on the slow sequence ("Conv."), slower than the converged sequence ("Slow"), more rapid than the converged sequence ("Rapid"), has a lower mass ("LM") than the converged sequence limit, or is too warm to efficiently spin down ("Warm"). notes on particular stars. Six stars appear to rotate more slowly than the bulk of the sample. Blending is not a concern for these stars (i.e., none have bright neighbors in DR2 within 1. ′ 5), their spot-modulated light curves show unambiguous periodicity, and they do not appear to be binaries according to their photometry, RV errors (σ < 2 km s −1 ), and kinematics. It is unclear to us why they are outliers. This work has made use of data from the European Space Agency (ESA) mission Gaia, 11 processed by the Gaia Data Processing and Analysis Consortium (DPAC). 12 Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.   2017) temperature (T eff = 3050 ± 50 K) and luminosity, corrected with the Gaia DR2 parallax (log L/L⊙ = −2.59 ± 0.05 dex, we infer a mass MB = 0.12 ± 0.01 M⊙ (≈123 MJup). This is greater than the hydrogen-burning limit and indicates that HD 1160 B is probably a very-low-mass star and not a brown dwarf.