Lodén 1 debunked

Sep 18, 2016

by Eunkyu Han

SALT helps proving the putative old and nearby cluster Lodén 1 to be neither old, nor nearby, nor a cluster.


Aerial view of SALT in Sutherland.

Stellar clusters are the keys to study stellar evolution and structure. By characterizing stellar
clusters, we understand how the age, mass, and metallicity affect the formation and evolution of
stars. For instance, stellar clusters with similar properties can demonstrate how uniformly
Lithium and magnetic activity decay. Measuring the abundances of Lithium gives us important
clues about the evolution of stars and their interior structures. In order to study these, we need
clusters that are close enough to be able to measure activity diagnostics—like the Ca II H & K
lines—and determine accurate Lithium abundances.

We have access to few benchmark clusters, nearly all of which are either nearby & young (e.g.
Pleiades, Hyades, and Praesepe, which are within 200 pc of Earth with the ages of 130 to 625
Myr) or distant & old (e.g. M67, which is ~1 kpc away with the age of 4 Gyr). However, we lack
old & nearby clusters that can link between young, nearby clusters and older, more distant
clusters in studies of stellar evolution. In 2013, Curtis et al. confirmed that Ruprecht 147 to be
the oldest and nearby cluster with an age of 3 Gyr and a distance of 300pc. While Curtis was
studying Ruprecht 147, he noticed 2 more unstudied open clusters that are possibly nearby and
old: Lodén 1 and NGC 2240.

Lodén 1’s first mention was made by a Swedish astronomer Lodén in 1980 where he spotted a
loose grouping of stars located between the constellations Vela and Carina in the Southern sky.
In his paper, Lodén claimed “evident concentration of late­type stars…and main­sequence A­F
stars,” and whose physical association was “not confirmed but strongly suspected”. Then in
2005, Karchenko et al. performed an automated search of 520 galactic clusters and determined
their membership as well as their properties based on the proper motions and the photometry.
Lodén 1 was one of the many open clusters determined by him with a reported age of 2 Gyr and
a distance of 360 pc. If this is true, Lodén 1 could become a useful benchmark cluster.
However, we were suspicious about Lodén 1 being an old and nearby cluster. Firstly, among the
memberships determined by Lodén in 1980, the two stars with well­ measured proper motions
show very low space motion which made us to think that they are not really as close as 300pc.
Secondly, putative members identified by Karchenko do not have proper motions that distinguish them from the field.

We attempted to determine the membership properties of Lodén 1 to confirm whether Lodén 1
is a real open cluster and—if so— establish its benchmark status by means of measuring radial
velocity. We collected medium resolution spectra of the candidate stars using SALT’s Robert
Stobie Spectrograph (RSS) to determine the star’s radial velocity (RV). The figure shows one of
the spectra we collected.


We cross­-correlated the candidates’ spectra with the spectra of RV standard stars to measure
the RVs of the candidates. The result is shown in the figure below where the RSS
measurements are in black and the RVs from a simulated set of field stars are in gray. Although
there is a peak near 12 km/s, this matches with the peak of the random stars in the background


We also assembled archival astrometry (PPMXL) and photometry (2MASS, Tycho­2, APASS),
neither of which revealed any sign of a cluster main­sequence turnoff or red giant branch among
all stars in the field brighter than J < 11.


Considering the 29 stars identified by Lodén and listed on SIMBAD as the members of Lodén 1,
we found no compelling evidence of kinematic clustering in proper motion or radial velocity.
Most of these candidates are A stars and red giants, which means they are not old. Their proper
motions are consistent with distant field stars in the Galactic plane (shown in the figure below),
meaning that they are not nearby. These facts, combined with their lack of distinct or common
space motions means these stars are not physically associated.


Thus, we can conclude that the old nearby cluster Lodén 1 is neither old, nor nearby, nor a cluster.

Link to the paper: http://dx.doi.org/10.3847/0004-6256/152/1/7.