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Organic chemistry around young high-mass stars Allen, Veronica Amber

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Allen, V. A. (2018). Organic chemistry around young high-mass stars: Observational and theoretical.

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Chapter 7

Additional sections

7.1 English Summary

Star formation

In our galaxy, as in others, there are stars of different sizes and masses which might all look the same, but in reality can be very different. If you look into the night sky in the winter, you may recognize the constellation of Orion which contains a number of stars much bigger than the sun, the most prominent of which is Rigel, a bright blue star at Orion’s foot. It is generally believed that stars begin to form when clouds of gas and dust are pulled together by gravity and that this material will become a flattened disk which funnels material to the young star (protostar) with gaps at the poles for energy to escape in outflows. Astronomers consider stars at least eight times more massive (heavier) than our sun to be high-mass stars and have been confounded by such stars for many years as it is not clear how they could form.

Observational Astrochemistry

High-mass protostars in particular are difficult to study because we

cannot observe them with optical light – they are hidden by the dust sur-

rounding them as they form. If we observe them at longer wavelengths

(like infrared) we can see through the dust, or even see the dust directly

and determine its properties. My observations generally use long wave-

lengths in the sub-millimeter and millimeter range (0.8-1.2 mm) where

Figure 7.1.1: The constellation Orion with Rigel in the bottom right. (Image credit

Derrick Lim from APOD: https://apod.nasa.gov/apod/ap180321.html).

7.1 English Summary

. . . . These lines are the light emitted from molecules when there is a change in the energy stored in them, sometimes from the bonds between atoms bending or stretching (vibrational transitions) or from a change in the rotation speed of the molecule (rotational transitions). Each of these energy changes is specific to the molecule that it arises from, so if several transitions are observed from a single molecule, you can confirm that molecule is in the gas that you are observing.

Figure 7.1.2: Diagram of vibrational and rotational motions that emit light as spectral lines. (from: https://bit.ly/2JHZiTL).

The shape of the spectral lines contains information about the move-

ment of the gas using the Doppler effect – when it is moving away from

you it becomes red-shifted (the line is at a longer wavelength) and

when it is moving toward you it is blue-shifted (the line is at a shorter

wavelength). By judging several lines from the same molecular species

Figure 7.1.3: Two ways of viewing spectral lines in optical light: as colored stripes of light from the spectrometer or as spectral lines showing brightness vs. wavelength.

containing this species – temperature, density, abundance (or concen- tration), and whether the motion is coordinated, for example. Some observations have maps, or images of the region where each pixel in the map has a spectrum filled with different lines. Using the maps you can follow the movement of different species if the spectral lines have different velocities at different pixels. In this way, you can see if the gas containing specific molecules is moving in a coordinated rotational way or in a more random way and you can compare the movement of different molecular species to see if they might be in the same gas.

Astrochemical Modeling

After determining the chemical composition of the gas using spectral

lines, we can use astrochemical models – calculations of chemistry

under specific conditions – to determine the ages of the young stars and

the physical properties affecting the gas. For example, more incoming

cosmic-rays (high energy particles) cause changes in the chemical re-

actions because they travel deep into the star-forming clouds and ionize

molecules within the gas or frozen onto dust grains. UV photons from

the young stars can also affect the chemistry in shallower layers of the

cloud or near the outflows. The type of models used in my work are based

on rate-equations, where the fractional amount of each molecule is cal-

culated based on the rate of its formation and destruction in chemical

reactions. These rates are affected by temperature and density among

7.1 English Summary

. . . .

Figure 7.1.4: Diagram showing how spectral lines show the movement of gas in a disk. Red-shifted parts of the disk are moving toward the observer and blue-shifted parts are moving away. This gives the effect of a double-peaked spectral line. (From the thesis of Rosina P. Hein Bertelsen).

other things so by changing these we can determine which physical con- ditions are important in our star-forming region.

In the model I use, the temperature changes over time and I slowly warm up dense gas from cold to hot temperatures in a manner expected of gas near a protostar that is growing and heating up its surroundings.

By starting at very cold temperatures (10 K), we have many different

molecular species frozen onto dust grains where they can react to form

more and more complex molecules. Not all molecules are formed mainly

on dust grains, some are formed in the gas and we can see this us-

ing chemical models. In my models, molecules like methanol (CH ₃ OH)

and ethanol (C ₂ H ₅ OH) are made mostly on grain surfaces while methyl

cyanide (CH ₃ CN) is formed in the ice and the gas. If the molecule abun-

from observations at a certain time then we know the protostar’s ap- proximate age and environmental conditions.

This thesis

In chapter 2, we determined the chemical composition of four young high-mass stars in the star-forming region G35.20-0.74N (G35.20 A, B1, B2, and B3) and one in G35.03+0.35 (G35.03 A) using spectral lines and maps obtained using ALMA (Atacama Large Millimeter Array), an array of more than 60 connected telescopes located in northern Chile.

Through this analysis, we discovered a difference in chemistry within G35.20 B across three sub-sources that appear to be within the same disk. The gas near sub-source B3 has 100x greater abundance of cyanides (molecules with CN) than B1 and B2. This implies that there are mul- tiple young stars within the disk-like structure. In comparing G35.20 A and G35.03 A we see that they have a very similar chemical composi- tion, but G35.03 A has no molecules containing deuterium (hydrogen + 1 neutron) whereas G35.20 A has a high percentage of molecules with deu- terium. The presence of deuterium implies that a source is very young, so this shows that G35.03 A is an older source.

In chapter 3, we try to understand the chemical differences in G35.20 B by using chemical models to reproduce these differences. Starting with a point deep inside the gas (so as to be unaffected by the UV photons from the young star), we increase the temperature from 10 to 500 K over different time periods. Slow warm up times are related to a low-mass star warming its surroundings as it forms and fast warm up times relate to a high-mass star. By testing the effect of changing different values for starting and ending temperature, gas density, starting ice composition, and cosmic-ray ionization rate we can pinpoint the physical conditions that cause the difference. The key for this scenario turns out to be a higher cosmic-ray ionization rate where with a slightly higher rate, we can reproduce the abundances of G35.20 B3 and those of B1 and B2 with the same physical conditions and a small (∼2000 year) age difference. The complex organic molecule ethyl cyanide (C ₂ H ₅ CN) was the key difference between the different sub-sources in G35.20 B, so we conclude that this molecule is a chemical clock, whose intensity can tell you about the age of a young high-mass star.

In chapter 4, we detail how we searched for the outflows expected

to be associated with G35.20 and G35.03. With new observations from

7.1 English Summary

. . . .

Figure 7.1.5: Art showing the dust from G35.20 (green lines) and the nebula created

by its outflows (orange).

NOEMA (the Northern Extended Millimeter Array), an array of 7 con- nected telescopes near Grenoble, France plus a large single dish telescope near Granada, Spain, we observed SiO and HCO ⁺ spectral lines, which can trace the outflow activity. We detected two separate outflows in G35.20 which appear to be associated with the sources in B, one of which is perpendicular to the motion of the gas. Detecting these out- flows is evidence supporting the conclusion that G35.20 B contains a disk surrounding multiple young high-mass stars. On the other hand, the outflows near with G35.03 are not clearly associated with G35.03 A, which may have a disk.

Figure 7.1.6: Sketch showing the formation of formamide on dust grains from HNCO.

In chapter 5, we study the formation of formamide (NH ₂ CHO), which is an important molecule to study because it has a similar molecular structure to amino acids and proteins so may be a precursor to life.

Formamide has been detected in many different environments includ-

ing high- and low-mass star-forming regions, outflows, and in the ice of

comets. There is a disagreement between astrochemists whether inter-

stellar formamide is dominantly formed in hot gas from reactions with

formaldehyde (H ₂ CO) or on dust grains in ice by adding hydrogen atoms

7.1 English Summary

. . . .

Figure 7.1.7: Sketch showing the formation of formamide in warm gas from H

CO.

to isocyanic acid (HNCO). This chapter is an observational experiment

to see if one of these formation pathways is dominant in six young O-type

(very high-mass) sources. Using ALMA maps we compare the physical

extent of the emission from these three key species, along with their ve-

locity structure and order of motion. Using spectral analysis we compare

the abundances of each species to each other and find a correlation be-

tween HNCO and formamide. The map analyses do not lead to an overall

clear dominant precursor, but in two sources HNCO and formamide are

consistently more similar to each other.

7.2 Nederlandse samenvatting

Stervorming

In ons sterrenstelsel, zoals in anderen, zijn sterren van verschillende groottes en massa’s die ondanks dat ze zeer op elkaar lijken, in werke- lijkheid erg verschillen. Als u in de winter naar de nachthemel kijkt, kunt u wellicht het sterrenbeeld Orion herkennen. Orion bevat een aan- tal sterren die velen malen groter zijn dan de zon, waarvan de meest prominente Rigel is, een heldere blauwe ster bij Orion’s voet. Over het algemeen gelooft men dat stervorming begint met het door zwaartekracht gedreven samentrekken van een wolk van gas en stof en dat dit materiaal een platte schijf vormt die meer materiaal naar de jonge ster (protoster) toe overbrengt met gaten bij de polen voor energie om te ontsnappen in de vorm van uitvloeiingen. Sterrenkundigen beschouwen sterren met ten minste acht keer de massa van de zon als hoge-massa sterren en zijn voor vele jaren verward door dit soort sterren gezien het nog niet duidelijk is hoe dit soort hoge massa sterren vormen.

Observationele astrochemie

Hoge-massa protosterren in het bijzonder zijn moeilijk te bestuderen omdat we ze niet kunnen observeren met optisch licht - ze zijn verborgen door omringend stof tijdens de formatie. Als we ze observeren met lan- gere golflengtes (zoals infrarood) kunnen we door dit stof heen kijken, of zelfs het stof zelf direct waarnemen en zijn eigenschappen bepalen.

Mijn observaties gebruiken over het algemeen lange golflengtes in het sub-millimeter en millimeter domein (0.8-1.2 mm) waar we koud stof en de vingerafdruk van moleculen kunnen zien in de vorm van spectraalli- jnen. Deze lijnen zijn het uitgestraalde licht van moleculen zodra zij een verandering ondervinden in de in hun interne energie, door het buigen of rekken van de verbindingen tussen atomen (vibrationele transities) of van een verandering in de rotatiesnelheid van het molecuul (rotationele transitie). Ieder van deze veranderingen in energie is specifiek voor het molecuul waar het uit voort komt, dus als meerdere transities van een molecuul geobserveerd zijn, kunt u bevestigen dat dit molecuul zich in het geobserveerde gas bevint.

De vorm van de spectraallijnen bevat informatie over de beweging van

het gas dankzij het Doppler effect - als de bron van u af beweegt wordt

de lijn naar langere golflengtes verschoven (roodverschuiving) en in-

7.2 Nederlandse samenvatting

. . . .

Figure 7.2.1: Het sterrenbeeld Orion met rechts onder de ster Rigel. (Krediet Derrick

Lim from APOD: https://apod.nasa.gov/apod/ap180321.html).

Figure 7.2.2: Diagram van de vibrationele en rotationele bewegingen die licht uit- stralen als spectraallijnen. (Van: https://bit.ly/2JHZiTL).

dien de bron naar u toe beweegt wordt de lijn naar kortere golflengtes verschoven (blauwverschuiving). Door meerdere spectraallijnen van dezelfde moleculaire soort samen te onderzoeken, is het vaak mogelijk om de eigenschappen van het gas waar deze soort zich in bevindt te be- grijpen, bijvoorbeeld de temperatuur, dichtheid, abondantie (oftewel concentratie), en de al dan niet gecoördineerde beweging van het gas.

Sommige observaties bevatten kaarten (maps) of afbeeldingen van de re-

gion waar iedere individuele pixel op de kaart een spectrum heeft dat is

gevuld met verschillende lijnen. Met behulp van deze kaarten is het mo-

gelijk de beweging van verschillende soorten moleculen te volgen, zolang

de spectraallijnen verschillende snelheden hebben op verschillende pix-

els. Op deze manier kunt u zien of het gas dat deze moleculen bevat op

gecoördineerde manier roteert, of dat het gas op een meer willekeurige

7.2 Nederlandse samenvatting

. . . .

Figure 7.2.3: Twee manieren om spectraallijnen in optische licht te bepalen: in de vorm van de gekleurde strepen van een spectrometer of als spectraallijnen in een grafiek waarin helderheid is uitgezet tegen golflengte.

manier beweegt. Verder kunt u ook de beweging van verschillende molec- ulaire soorten vergelijken om te zien of ze zich in hetzelfde gas bevinden.

Astrochemische modellering

Na het bepalen van de chemische compositie van het gas met behulp

van spectraallijnen kunnen we astrochemische modellen - berekenin-

gen van chemie onder specifieke condities - gebruiken om de leeftijd van

de jonge sterren en de fysische eigenschappen die het gas beïnvloeden

te bepalen. Een toename in kosmische straling (hoog-energetische

deeltjes) veroorzaakt bijvoorbeeld veranderingen in de chemische reac-

ties omdat ze diep doordringen in de stervormende nevel en moleculen

ioniseren in het gas of op stofdeeltjes. UV-fotonen van de jonge sterren

kunnen ook de chemie in ondiepe lagen van de nevel of bij uitvloeiingen

beïnvloeden. Het type model gebruikt in mijn werk is gebaseerd op re-

actiesnelheids afhankelijke vergelijkingen, waar de fractionele hoeveel-

heid van elk molecuul wordt berekend op basis van de snelheid waarmee

het gevormd en vernietigd wordt in chemische reacties. Deze vergeli-

jkingen worden beïnvloed door onder andere temperatuur en dichtheid,

dus door deze factoren te veranderen kunnen we bepalen welke fysische

condities belangrijk zijn in onze stervormende regio’s. In het model dat

ik gebruik, verandert de temperatuur in de loop van de tijd en warm ik

Figure 7.2.4: Dit diagram laat zien hoe spectraallijnen de beweging van gas in een schijf weergeven. (Uit het proefschrift van Rosina P. Hein Bertelsen).

Dit gebeurt op een manier die verwacht wordt van gas dat verwarmd wordt in de buurt van een groeiende protoster. Door te beginnen met zeer koude temperaturen (10 K) hebben we veel verschillende moleculaire soorten vastgevroren op stofdeeltjes, waar ze kunnen reageren om meer en meer complexere moleculen te maken. Niet alle moleculen worden hoofdzakelijk gevormd op stofdeeltjes, sommigen worden gevormd in het gas en we kunnen dit zien met behulp van chemische modellen. In mijn model worden moleculen zoals methanol (CH ₃ OH) en ethanol (C ₂ H ₅ OH) hoofdzakelijk op het oppervlak van stofdeeltjes gemaakt terwijl methyl- cyanide (CH ₃ CN) hoofdzakelijk in het ijs en in het gas gemaakt wordt.

Als de abondanties van moleculen op een bepaalde tijd van het model

met specifieke omstandigheden overeenkomen met de abondanties uit

de waarnemingen dan weten we de geschatte leeftijd en omgevingsom-

standigheden van de protoster.

7.2 Nederlandse samenvatting

. . . .

Dit proefscrift

In hoofdstuk 2 bepalen we de chemische compositie van vier sub- bronnen in de stervormende regio G35.20-0.74N (G35.20 A, B1, B2, and B3) en één in G35.03+0.35 (G35.03 A) met behulp van spectraallijnen en kaarten verkregen met behulp van ALMA (Atacama Large Millime- ter Array), een reeks van meer dan 60 verbonden telescopen in noord Chili. Door deze analyse ontdekten we een verschil in de chemie binnen G35.20 B over drie sub-bronnen die lijken binnen één schijf te zitten. Het gas in de buurt van sub-bron B3 heeft een 100 keer grotere abondantie van cyanides (moleculen met CN) dan B1 en B2. Dit impliceert dat er meerdere jonge sterren in de schijfachtige structuur zitten. Door G35.20 A en G35.03 A te vergelijken zien we dat ze een zeer gelijke chemische compositie hebben, maar dat G35.03 A geen moleculen met deuterium (waterstof + 1 neutron) bevat terwijl G35.20 A een zeer groot percentage moleculen met deuterium heeft. De aanwezigheid van deuterium geeft aan dat de bron nog zeer jong is. Dit laat dus zien dat G35.03 A een oudere bron is.

In hoofdstuk 3 proberen we de chemische verschillen in G35.20 B

te begrijpen door met behulp van chemische modellen deze verschillen

te reproduceren. Beginnende met een punt diep in het gas (om niet

te worden beïnvloed door de UV-fotonen van de jonge ster), verhogen

we de temperatuur van 10 to 500K gedurende verschillende tijdsperio-

den. Trage opwarmtijden worden geassocieerd aan een lage-massa ster

die zijn omgeving opwarmt tijdens zijn formatie en snelle opwarmtijden

worden geassocieerd aan een hoge-massa ster. Door het effect te testen

van het veranderen van verschillende waarden voor begin- en eindtem-

peratuur, gasdichtheid, startcompositie van het ijs en ionisatiesnelheid

van kosmische straling kunnen we de fysische condities die het verschil

veroorzaken vaststellen. De sleutel voor deze situatie bleek een hogere

ionisatiesnelheid door de kosmische straling te zijn, waar met een iets

hogere snelheid we de chemische abondanties van G35.20 B3 en die van

B1 en B2 kunnen reproduceren met dezelfde fysische condities en een

klein leeftijdsverschil (∼2000 jaar). Het complexe organische molecuul

ethylcyanide (C ₂ H ₅ CN) was het belangrijkste verschil tussen de verschil-

lende sub-bronnen in G35.20 B. Hieruit concludeerde we dat dit molecuul

een chemische klok is, waarvan de detectie of non-detectie u kan vertellen

Figure 7.2.5: Impressie die het stof van G35.20 (groene lijnen) en de nevel gemaakt

door zijn uitstromen (oranje) laat zien.

7.2 Nederlandse samenvatting

. . . . In hoofdstuk 4 detailleren we hoe we gezocht hebben naar de verwachte uitvloeiingen geassocieerd met G35.20 en G35.03. Met nieuwe obser- vaties van NOEMA (de Northern Extended Millmeter Array), een reeks van 7 verbonden telescopen in de buurt van Grenoble, Frankrijk, en een grote telescoop in de buurt van Granada, Spanje, observeerden we de spectraallijnen van SiO en HCO ⁺ die de uitstroomactiviteit kunnen vol- gen. We hebben twee afzonderlijke uitstromen gedetecteerd in G35.20 die lijken geassocieerd te zijn met de bronnen in B, waarvan er één loodrecht staat op de beweging van het gas. Het detecteren van deze uitstroom is ondersteunend bewijs voor de conclusie dat G35.20 B een schijf bevat die meerdere jonge hoge-massa sterren omringt. Aan de andere kant zijn de uitstromen in de buurt van G35.03 niet duidelijk geassocieerd met G35.03 A, die mogelijk een schijf heeft.

Figure 7.2.6: De formatie van formamide uit HNCO op stofdeeltjes weergegeven in een schets.

In hoofdstuk 5 bestuderen we de vorming van formamide (NH ₂ CHO),

dat een belangrijk molecuul is om nader te bekijken omdat het een

vergelijkbare moleculaire structuur heeft als aminozuren en eiwitten en

Figure 7.2.7: De formatie van formamide uit H

CO in warm gas weergegeven in een schets.

schillende omgevingen waargenomen, waaronder stervormende regio’s

met hoge en lage massa, uitstromen en in het ijs van kometen. Er is

een meningsverschil tussen verschillende astrochemici of interstellaire

formamide hoofdzakelijk wordt gevormd in heet gas uit reacties met

formaldehyde (H ₂ CO) of op stofkorrels in ijs door toevoeging van wa-

terstofatomen aan isocyaanzuur (HNCO). Dit hoofdstuk is een obser-

vationeel experiment om te zien of een van deze formatieroutes domi-

nant is in zes jonge O-type (zeer hoge massa) bronnen. Met behulp van

ALMA-kaarten vergelijken we de fysieke omvang van de emissie van deze

drie belangrijkste soorten, samen met hun snelheidsstructuur en volgo-

rde van beweging. Met behulp van spectrale analyse vergelijken we de

abondanties van elke soort met elkaar en vinden een correlatie tussen

HNCO en formamide. De analyses van de kaarten leiden niet tot een

algemene duidelijke dominante voorloper, maar in twee bronnen lijken

HNCO en formamide meer gelijk aan elkaar.

7.3 Acknowledgements

. . . .

7.3 Acknowledgements

Scientific

To my supervisor, Floris, thank you so much for choosing me for your position. It is here that I have fallen in love with astrochemistry and discovered that this is a job that I can do. I am grateful for your guidance and support. I also greatly appreciate your patience, we didn’t always understand each other, but we found our rhythm in the end.

Before I came to Kapteyn, I never really found belonging where I was, but the welcoming and sociable atmosphere here made me imme- diately feel at home. As in all academic settings, this family has had many faces over the years, and I have had many new "sisters" (and a few "brothers") and I’ll honor as many as possible here.

To my first Kapteyn best friend, Rosina, you showed me the way of the PhD astronomom and proved that it was possible to do it all. You opened your homes to us all (in Ten Boer and in Copenhagen) for beautifully cooked dinners, sleepovers, and Toby’s endless joking. I’m so happy our families could become friends, even if it meant I had to listen to ’Let it go’ a million times.

To Mariya, you showed me how to be the strongest woman in science possible, while also being allowed to cry. If anyone has been my mentor in my time here, it is you. Thank you for all of the coffees and ice skating sessions and playdates.

To Margot, my newest best friend, just when I had started feeling alone after all of my previous friends were gone, you arrived and suddenly I could never be lonely again. I am amazed by your generosity and kind- ness and so glad of all the conversations we had in my last year. I can never repay you for the help you gave my family on our last day in the Netherlands but I have definitely learned never to try to take 16 suitcases on an airplane again.

To Evgenia, first my groupmate and now one of my best friends, I am so thankful for all of great times we have had both at home and abroad.

Sitting by the Mediterranean deep into the night just talking about life

made for one of the most rejuvenating weeks of my life. You were always

one of the few people that I could talk with about both science and re-

lationships.

the many who have gone before, thank you for your scientific (and non- scientific) conversations over the years. You could never just be col- leagues for me.

Shoko, you have been the one real constant during my time as a PhD student. From my first day when I saw you on the couches with your eye injury and thought you seemed to be a kindred spirit, to moving into my new house and laying flooring together, to my last days when we were both desperately packing you have been there as a true friend. For all of the lunches we had and all of your help in my job search and general chats about life, I am so glad to know you. Also, this is probably full of run-on sentences but knowing your faults is the first step to improving, right?

Simon, I’ve really enjoyed your company over the past couple of years talking about Star Wars and DnD and nonsense. I am so grateful for your (and Jose’s!) hard work and expertise in translating my summary.

Bertrand, thank you for late night WhatsApp chats and for introducing me to Hero Quest including the tiny furniture and the broadsword!

Thank you to Caroline van Borm for her layout design and LaTeX help, to Alexi Elconin for designing the butterfly nebula art that I have used on so many posters, and to Nick Oberg for creating some amazing cover art for me.

To Scott Trager, before I came to Kapteyn I generally avoided other Americans as they were never quite my kind of people, so I was astounded to find another wayward American who actually cares about things like diversity and supporting marginalized people. I’m glad you’ve reached a position where you can use your powers for good. When you helped me with a confrontation a few years ago, I didn’t think I could trust a man to help and I’m glad to find I was wrong.

Mariano, my PBC mentor, and Lucia, you know the whole story of one problem after another just associated with my ridiculous life and man- aged to find so many solutions. I am grateful for you and the support of the institute.

To Peter Barthel, technically my second supervisor, you did not need

to do so much supervising, but I am so glad that you advocated for me

with the university when things were getting bad. We could afford to

buy food in my second year because of you. You have also been an in-

spiration in public outreach and education.

7.3 Acknowledgements

. . . . Christa and the other office staff, I am so glad for your help and our shared conversations over the years. I know that you are all so much more than the secretariat and I hope that no one ever forgets it.

To my collaborators around the world: Riccardo, Luke, Katharine, Stu- art, Melvin, Joe, Ana, Timea, I have learned so much from you all and I hope we can continue working together in the future. Álvaro, I have been asking you questions for 4 years now and you never treated me like an annoying student. Thank you for being like my unofficial extra su- pervisor. Catherine, thank you for introducing me to chemical modeling – I am complete as a scientist now. Your patient explanations about as- trochemistry and letters of support in my pursuit of a postdoc position are priceless.

René Oudmaijer, though we have not collaborated during my PhD, I have enjoyed our conversations in my visits to Leeds and at conferences.

Hanging out with you, Alice, Miguel, Abi, Rob, and the rest of the Leeds crew reminded me that strong friendships can be built across the world.

Never stop being a cool professor.

Outside science

To Evelyn’s teachers, juf Greetje, Eveline, and Nadine (plus all of the teaching assistants and trainees) – you are irreplaceable! You do the most important work in society and somehow control dozens of small kids (I can’t even control one). Thank you so much for the influence you have had on Evelyn and your connection and support of our family. Also a huge thank you to Juf Natasja, who gave me my first opportunity to inspire children to love the universe. My Dutch wasn’t perfect but I am so proud to have had an impact on those kids.

To the leidsters at Kits Kinderdagverblijf, Irene, Janine, Manon, Geri- anne, Jannie, Anneke, and so many more – I could always tell that you loved my kids as much as I do and I could not have done my job if you hadn’t done yours. Kits was their second home and the fact that they loved you so much made our daily separations so much easier. Thank you so much!

To Linda, my thuisbegeleider, I could not have gotten though this last

year without you and I regret not asking for help sooner. Your job is

amazing and I don’t doubt that you save lives. Good luck with your new

baby and welcome to the parents club.

while I was traveling – Ben Sanderson, Natalie and Lewis, Mat’s family – I could not have done my job without your help and I know that you made the kids so happy with your visits. To Natalie in particular, you are one of my best friends, my paranymph, and a wonderful influence on my kids. You have seen Evelyn’s first steps and her first time trying ice hockey. You are a badass, hardcore woman who I never want to be without. We could never have finished moving from the Netherlands without you and your determination. I am sad that we will see less of each other now, but we can always find a way.

Mom, you have had a weird past few years but I’m so amazed by who you have become in your independence. Thank you for coming to help me take care of my little baby and thank you for moving to the "cold"

of Maryland to continue to support us while I follow my dreams. You are amazing. Never let anyone tell you otherwise. My sister, Ali, I am so glad we had the chance to reconnect and I’m excited that you want to be part of my family.

For my big kid Evelyn, Little Love – you are so much like me its terrifying. I can’t believe how grown up you are and you’re only 6. I have never been so happy to hear that someone was proud of me and you tell me all the time! You have traveled around the world with me, my little astronomy mascot, and helped me get other kids interested in astronomy and science. I am constantly amazed by you. I have watched you become so cool and smart and caring. The American kids aren’t going to know what hit them when we get there. You were always the coolest kid I’d ever met and I hope you continue to grow into an amazing person.

To my baby, Tristan – you are too little to understand, but I’m so lucky to have you as my PhD baby! You are so cheerful and easygoing that you make everyone who sees you happy. I am so excited to see who you will become as you navigate the world.

To my partner-in-crime, Mathew – I bet when you met me 12 years ago

you didn’t think I’d be dragging you all over the world! In all the lives

we’ve had, the ups and downs and wtfs, this has been the best so far,

and then next one will be better yet. We started at the bottom and now

we’re a little bit above the bottom. I don’t know how you got to know

me better than I know myself, but I always appreciate the reflection and

understanding you can provide. Thank you for coming with me on my

7.3 Acknowledgements

. . . . crazy journey and helping in every way you can.

Overall, I’m thankful to the governments and societies of the UK and the Netherlands that choose to be supportive of people who are struggling. If I had gone back to the US 13 years ago like I was supposed to I would never have achieved my dream. Now that I am returning after almost 15 years in Europe to take up a postdoc position at NASA, my childhood dream has come true, but it was not the American system that supported me. It was the UK universities not kicking me out when I missed deadline after deadline when was a new parent with a sick baby.

It was professors and other students who were happy to look after a little baby for an hour while I was doing my final masters assessments.

It was the NWO who pays PhD students as if they are intelligent human beings and not just disposable computer monkeys. It was the University of Groningen who found funds for me to travel to work when I couldn’t afford food or bills. It was gemeente Hoogezand-Sappemeer who saw our family’s struggles and found us solutions and assistance to get through these years. It was the Dutch government who gives tax credits to people who can’t really afford rent, healthcare, or childcare and who allows you to take time off work to be with your kids without a deep paycut and dire consequences. It was the PBC who did an amazing job inspiring me to stay on track and never judged me unworthy because of my problems.

I was raised to think that if you couldn’t achieve success without some

kind of help then you were somehow a failure, but I am proud to say

that I had the help of these people and systems to achieve so much more

than I could have alone.

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